dibbler/sqlalchemy/orm/session.py

# session.py
# Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Michael Bayer mike_mp@zzzcomputing.com
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""Provides the Session class and related utilities."""

import weakref
from itertools import chain
import sqlalchemy.exceptions as sa_exc
from sqlalchemy import util, sql, engine, log
from sqlalchemy.sql import util as sql_util, expression
from sqlalchemy.orm import (
    SessionExtension, attributes, exc, query, unitofwork, util as mapperutil, state
    )
from sqlalchemy.orm.util import object_mapper as _object_mapper
from sqlalchemy.orm.util import class_mapper as _class_mapper
from sqlalchemy.orm.util import (
    _class_to_mapper, _state_has_identity, _state_mapper,
    )
from sqlalchemy.orm.mapper import Mapper, _none_set
from sqlalchemy.orm.unitofwork import UOWTransaction
from sqlalchemy.orm import identity

__all__ = ['Session', 'SessionTransaction', 'SessionExtension']


def sessionmaker(bind=None, class_=None, autoflush=True, autocommit=False,
                 expire_on_commit=True, **kwargs):
    """Generate a custom-configured :class:`~sqlalchemy.orm.session.Session` class.

    The returned object is a subclass of ``Session``, which, when instantiated
    with no arguments, uses the keyword arguments configured here as its
    constructor arguments.

    It is intended that the `sessionmaker()` function be called within the
    global scope of an application, and the returned class be made available
    to the rest of the application as the single class used to instantiate
    sessions.

    e.g.::

        # global scope
        Session = sessionmaker(autoflush=False)

        # later, in a local scope, create and use a session:
        sess = Session()

    Any keyword arguments sent to the constructor itself will override the
    "configured" keywords::

        Session = sessionmaker()

        # bind an individual session to a connection
        sess = Session(bind=connection)

    The class also includes a special classmethod ``configure()``, which
    allows additional configurational options to take place after the custom
    ``Session`` class has been generated.  This is useful particularly for
    defining the specific ``Engine`` (or engines) to which new instances of
    ``Session`` should be bound::

        Session = sessionmaker()
        Session.configure(bind=create_engine('sqlite:///foo.db'))

        sess = Session()

    Options:

    autocommit
      Defaults to ``False``. When ``True``, the ``Session`` does not keep a
      persistent transaction running, and will acquire connections from the
      engine on an as-needed basis, returning them immediately after their
      use. Flushes will begin and commit (or possibly rollback) their own
      transaction if no transaction is present. When using this mode, the
      `session.begin()` method may be used to begin a transaction explicitly.

      Leaving it on its default value of ``False`` means that the ``Session``
      will acquire a connection and begin a transaction the first time it is
      used, which it will maintain persistently until ``rollback()``,
      ``commit()``, or ``close()`` is called. When the transaction is released
      by any of these methods, the ``Session`` is ready for the next usage,
      which will again acquire and maintain a new connection/transaction.

    autoflush
      When ``True``, all query operations will issue a ``flush()`` call to
      this ``Session`` before proceeding. This is a convenience feature so
      that ``flush()`` need not be called repeatedly in order for database
      queries to retrieve results. It's typical that ``autoflush`` is used in
      conjunction with ``autocommit=False``.  In this scenario, explicit calls
      to ``flush()`` are rarely needed; you usually only need to call
      ``commit()`` (which flushes) to finalize changes.

    bind
      An optional ``Engine`` or ``Connection`` to which this ``Session``
      should be bound. When specified, all SQL operations performed by this
      session will execute via this connectable.

    binds
      An optional dictionary, which contains more granular "bind" information
      than the ``bind`` parameter provides. This dictionary can map individual
      ``Table`` instances as well as ``Mapper`` instances to individual
      ``Engine`` or ``Connection`` objects. Operations which proceed relative
      to a particular ``Mapper`` will consult this dictionary for the direct
      ``Mapper`` instance as well as the mapper's ``mapped_table`` attribute
      in order to locate an connectable to use. The full resolution is
      described in the ``get_bind()`` method of ``Session``. Usage looks
      like::

        sess = Session(binds={
            SomeMappedClass: create_engine('postgresql://engine1'),
            somemapper: create_engine('postgresql://engine2'),
            some_table: create_engine('postgresql://engine3'),
            })

      Also see the ``bind_mapper()`` and ``bind_table()`` methods.

    \class_
      Specify an alternate class other than ``sqlalchemy.orm.session.Session``
      which should be used by the returned class.  This is the only argument
      that is local to the ``sessionmaker()`` function, and is not sent
      directly to the constructor for ``Session``.

    _enable_transaction_accounting
      Defaults to ``True``.  A legacy-only flag which when ``False``
      disables *all* 0.5-style object accounting on transaction boundaries,
      including auto-expiry of instances on rollback and commit, maintenance of
      the "new" and "deleted" lists upon rollback, and autoflush
      of pending changes upon begin(), all of which are interdependent.

    expire_on_commit
      Defaults to ``True``. When ``True``, all instances will be fully expired after
      each ``commit()``, so that all attribute/object access subsequent to a completed
      transaction will load from the most recent database state.

    extension
      An optional :class:`~sqlalchemy.orm.session.SessionExtension` instance, or
      a list of such instances, which
      will receive pre- and post- commit and flush events, as well as a
      post-rollback event.  User- defined code may be placed within these
      hooks using a user-defined subclass of ``SessionExtension``.

    query_cls
      Class which should be used to create new Query objects, as returned
      by the ``query()`` method.  Defaults to :class:`~sqlalchemy.orm.query.Query`.

    twophase
      When ``True``, all transactions will be started using
      :mod:~sqlalchemy.engine_TwoPhaseTransaction. During a ``commit()``, after
      ``flush()`` has been issued for all attached databases, the
      ``prepare()`` method on each database's ``TwoPhaseTransaction`` will be
      called. This allows each database to roll back the entire transaction,
      before each transaction is committed.

    weak_identity_map
      When set to the default value of ``True``, a weak-referencing map is
      used; instances which are not externally referenced will be garbage
      collected immediately. For dereferenced instances which have pending
      changes present, the attribute management system will create a temporary
      strong-reference to the object which lasts until the changes are flushed
      to the database, at which point it's again dereferenced. Alternatively,
      when using the value ``False``, the identity map uses a regular Python
      dictionary to store instances. The session will maintain all instances
      present until they are removed using expunge(), clear(), or purge().

    """
    kwargs['bind'] = bind
    kwargs['autoflush'] = autoflush
    kwargs['autocommit'] = autocommit
    kwargs['expire_on_commit'] = expire_on_commit

    if class_ is None:
        class_ = Session

    class Sess(object):
        def __init__(self, **local_kwargs):
            for k in kwargs:
                local_kwargs.setdefault(k, kwargs[k])
            super(Sess, self).__init__(**local_kwargs)

        def configure(self, **new_kwargs):
            """(Re)configure the arguments for this sessionmaker.

            e.g.::

                Session = sessionmaker()

                Session.configure(bind=create_engine('sqlite://'))
            """
            kwargs.update(new_kwargs)
        configure = classmethod(configure)
    s = type.__new__(type, "Session", (Sess, class_), {})
    return s


class SessionTransaction(object):
    """A Session-level transaction.

    This corresponds to one or more :class:`~sqlalchemy.engine.Transaction`
    instances behind the scenes, with one ``Transaction`` per ``Engine`` in
    use.

    Direct usage of ``SessionTransaction`` is not necessary as of SQLAlchemy
    0.4; use the ``begin()`` and ``commit()`` methods on ``Session`` itself.

    The ``SessionTransaction`` object is **not** thread-safe.

    .. index::
      single: thread safety; SessionTransaction

    """

    def __init__(self, session, parent=None, nested=False):
        self.session = session
        self._connections = {}
        self._parent = parent
        self.nested = nested
        self._active = True
        self._prepared = False
        if not parent and nested:
            raise sa_exc.InvalidRequestError(
                "Can't start a SAVEPOINT transaction when no existing "
                "transaction is in progress")

        if self.session._enable_transaction_accounting:
            self._take_snapshot()

    @property
    def is_active(self):
        return self.session is not None and self._active

    def _assert_is_active(self):
        self._assert_is_open()
        if not self._active:
            raise sa_exc.InvalidRequestError(
                "The transaction is inactive due to a rollback in a "
                "subtransaction.  Issue rollback() to cancel the transaction.")

    def _assert_is_open(self, error_msg="The transaction is closed"):
        if self.session is None:
            raise sa_exc.InvalidRequestError(error_msg)

    @property
    def _is_transaction_boundary(self):
        return self.nested or not self._parent

    def connection(self, bindkey, **kwargs):
        self._assert_is_active()
        engine = self.session.get_bind(bindkey, **kwargs)
        return self._connection_for_bind(engine)

    def _begin(self, nested=False):
        self._assert_is_active()
        return SessionTransaction(
            self.session, self, nested=nested)

    def _iterate_parents(self, upto=None):
        if self._parent is upto:
            return (self,)
        else:
            if self._parent is None:
                raise sa_exc.InvalidRequestError(
                    "Transaction %s is not on the active transaction list" % (
                    upto))
            return (self,) + self._parent._iterate_parents(upto)

    def _take_snapshot(self):
        if not self._is_transaction_boundary:
            self._new = self._parent._new
            self._deleted = self._parent._deleted
            return

        if not self.session._flushing:
            self.session.flush()

        self._new = weakref.WeakKeyDictionary()
        self._deleted = weakref.WeakKeyDictionary()

    def _restore_snapshot(self):
        assert self._is_transaction_boundary

        for s in set(self._new).union(self.session._new):
            self.session._expunge_state(s)

        for s in set(self._deleted).union(self.session._deleted):
            self.session._update_impl(s)

        assert not self.session._deleted

        for s in self.session.identity_map.all_states():
            _expire_state(s, s.dict, None, instance_dict=self.session.identity_map)

    def _remove_snapshot(self):
        assert self._is_transaction_boundary

        if not self.nested and self.session.expire_on_commit:
            for s in self.session.identity_map.all_states():
                _expire_state(s, s.dict, None, instance_dict=self.session.identity_map)

    def _connection_for_bind(self, bind):
        self._assert_is_active()

        if bind in self._connections:
            return self._connections[bind][0]

        if self._parent:
            conn = self._parent._connection_for_bind(bind)
            if not self.nested:
                return conn
        else:
            if isinstance(bind, engine.Connection):
                conn = bind
                if conn.engine in self._connections:
                    raise sa_exc.InvalidRequestError(
                        "Session already has a Connection associated for the "
                        "given Connection's Engine")
            else:
                conn = bind.contextual_connect()

        if self.session.twophase and self._parent is None:
            transaction = conn.begin_twophase()
        elif self.nested:
            transaction = conn.begin_nested()
        else:
            transaction = conn.begin()

        self._connections[conn] = self._connections[conn.engine] = \
          (conn, transaction, conn is not bind)
        for ext in self.session.extensions:
            ext.after_begin(self.session, self, conn)
        return conn

    def prepare(self):
        if self._parent is not None or not self.session.twophase:
            raise sa_exc.InvalidRequestError(
                "Only root two phase transactions of can be prepared")
        self._prepare_impl()

    def _prepare_impl(self):
        self._assert_is_active()
        if self._parent is None or self.nested:
            for ext in self.session.extensions:
                ext.before_commit(self.session)

        stx = self.session.transaction
        if stx is not self:
            for subtransaction in stx._iterate_parents(upto=self):
                subtransaction.commit()

        if not self.session._flushing:
            self.session.flush()

        if self._parent is None and self.session.twophase:
            try:
                for t in set(self._connections.values()):
                    t[1].prepare()
            except:
                self.rollback()
                raise

        self._deactivate()
        self._prepared = True

    def commit(self):
        self._assert_is_open()
        if not self._prepared:
            self._prepare_impl()

        if self._parent is None or self.nested:
            for t in set(self._connections.values()):
                t[1].commit()

            for ext in self.session.extensions:
                ext.after_commit(self.session)

            if self.session._enable_transaction_accounting:
                self._remove_snapshot()

        self.close()
        return self._parent

    def rollback(self):
        self._assert_is_open()

        stx = self.session.transaction
        if stx is not self:
            for subtransaction in stx._iterate_parents(upto=self):
                subtransaction.close()

        if self.is_active or self._prepared:
            for transaction in self._iterate_parents():
                if transaction._parent is None or transaction.nested:
                    transaction._rollback_impl()
                    transaction._deactivate()
                    break
                else:
                    transaction._deactivate()

        self.close()
        return self._parent

    def _rollback_impl(self):
        for t in set(self._connections.values()):
            t[1].rollback()

        if self.session._enable_transaction_accounting:
            self._restore_snapshot()

        for ext in self.session.extensions:
            ext.after_rollback(self.session)

    def _deactivate(self):
        self._active = False

    def close(self):
        self.session.transaction = self._parent
        if self._parent is None:
            for connection, transaction, autoclose in set(self._connections.values()):
                if autoclose:
                    connection.close()
                else:
                    transaction.close()
            if not self.session.autocommit:
                self.session.begin()
        self._deactivate()
        self.session = None
        self._connections = None

    def __enter__(self):
        return self

    def __exit__(self, type, value, traceback):
        self._assert_is_open("Cannot end transaction context. The transaction was closed from within the context")
        if self.session.transaction is None:
            return
        if type is None:
            try:
                self.commit()
            except:
                self.rollback()
                raise
        else:
            self.rollback()

class Session(object):
    """Manages persistence operations for ORM-mapped objects.

    The Session is the front end to SQLAlchemy's **Unit of Work**
    implementation.  The concept behind Unit of Work is to track modifications
    to a field of objects, and then be able to flush those changes to the
    database in a single operation.

    SQLAlchemy's unit of work includes these functions:

    * The ability to track in-memory changes on scalar- and collection-based
      object attributes, such that database persistence operations can be
      assembled based on those changes.

    * The ability to organize individual SQL queries and population of newly
      generated primary and foreign key-holding attributes during a persist
      operation such that referential integrity is maintained at all times.

    * The ability to maintain insert ordering against the order in which new
      instances were added to the session.

    * An Identity Map, which is a dictionary keying instances to their unique
      primary key identity. This ensures that only one copy of a particular
      entity is ever present within the session, even if repeated load
      operations for the same entity occur. This allows many parts of an
      application to get a handle to a particular object without any chance of
      modifications going to two different places.

    When dealing with instances of mapped classes, an instance may be
    *attached* to a particular Session, else it is *unattached* . An instance
    also may or may not correspond to an actual row in the database. These
    conditions break up into four distinct states:

    * *Transient* - an instance that's not in a session, and is not saved to
      the database; i.e. it has no database identity. The only relationship
      such an object has to the ORM is that its class has a ``mapper()``
      associated with it.

    * *Pending* - when you ``add()`` a transient instance, it becomes
      pending. It still wasn't actually flushed to the database yet, but it
      will be when the next flush occurs.

    * *Persistent* - An instance which is present in the session and has a
      record in the database. You get persistent instances by either flushing
      so that the pending instances become persistent, or by querying the
      database for existing instances (or moving persistent instances from
      other sessions into your local session).

    * *Detached* - an instance which has a record in the database, but is not
      in any session. Theres nothing wrong with this, and you can use objects
      normally when they're detached, **except** they will not be able to
      issue any SQL in order to load collections or attributes which are not
      yet loaded, or were marked as "expired".

    The session methods which control instance state include ``add()``,
    ``delete()``, ``merge()``, and ``expunge()``.

    The Session object is generally **not** threadsafe.  A session which is
    set to ``autocommit`` and is only read from may be used by concurrent
    threads if it's acceptable that some object instances may be loaded twice.

    The typical pattern to managing Sessions in a multi-threaded environment
    is either to use mutexes to limit concurrent access to one thread at a
    time, or more commonly to establish a unique session for every thread,
    using a threadlocal variable.  SQLAlchemy provides a thread-managed
    Session adapter, provided by the :func:`~sqlalchemy.orm.scoped_session`
    function.

    """

    public_methods = (
        '__contains__', '__iter__', 'add', 'add_all', 'begin', 'begin_nested',
        'close', 'commit', 'connection', 'delete', 'execute', 'expire',
        'expire_all', 'expunge', 'expunge_all', 'flush', 'get_bind', 'is_modified', 
        'merge', 'query', 'refresh', 'rollback', 
        'scalar')

    def __init__(self, bind=None, autoflush=True, expire_on_commit=True,
                _enable_transaction_accounting=True,
                 autocommit=False, twophase=False, 
                 weak_identity_map=True, binds=None, extension=None, query_cls=query.Query):
        """Construct a new Session.

        Arguments to ``Session`` are described using the
        :func:`~sqlalchemy.orm.sessionmaker` function.

        """
        
        if weak_identity_map:
            self._identity_cls = identity.WeakInstanceDict
        else:
            self._identity_cls = identity.StrongInstanceDict
        self.identity_map = self._identity_cls()

        self._new = {}   # InstanceState->object, strong refs object
        self._deleted = {}  # same
        self.bind = bind
        self.__binds = {}
        self._flushing = False
        self.transaction = None
        self.hash_key = id(self)
        self.autoflush = autoflush
        self.autocommit = autocommit
        self.expire_on_commit = expire_on_commit
        self._enable_transaction_accounting = _enable_transaction_accounting
        self.twophase = twophase
        self.extensions = util.to_list(extension) or []
        self._query_cls = query_cls
        self._mapper_flush_opts = {}

        if binds is not None:
            for mapperortable, bind in binds.iteritems():
                if isinstance(mapperortable, (type, Mapper)):
                    self.bind_mapper(mapperortable, bind)
                else:
                    self.bind_table(mapperortable, bind)

        if not self.autocommit:
            self.begin()
        _sessions[self.hash_key] = self

    def begin(self, subtransactions=False, nested=False):
        """Begin a transaction on this Session.

        If this Session is already within a transaction, either a plain
        transaction or nested transaction, an error is raised, unless
        ``subtransactions=True`` or ``nested=True`` is specified.

        The ``subtransactions=True`` flag indicates that this ``begin()`` can
        create a subtransaction if a transaction is already in progress.  A
        subtransaction is a non-transactional, delimiting construct that
        allows matching begin()/commit() pairs to be nested together, with
        only the outermost begin/commit pair actually affecting transactional
        state.  When a rollback is issued, the subtransaction will directly
        roll back the innermost real transaction, however each subtransaction
        still must be explicitly rolled back to maintain proper stacking of
        subtransactions.

        If no transaction is in progress, then a real transaction is begun.

        The ``nested`` flag begins a SAVEPOINT transaction and is equivalent
        to calling ``begin_nested()``.

        """
        if self.transaction is not None:
            if subtransactions or nested:
                self.transaction = self.transaction._begin(
                    nested=nested)
            else:
                raise sa_exc.InvalidRequestError(
                    "A transaction is already begun.  Use subtransactions=True "
                    "to allow subtransactions.")
        else:
            self.transaction = SessionTransaction(
                self, nested=nested)
        return self.transaction  # needed for __enter__/__exit__ hook

    def begin_nested(self):
        """Begin a `nested` transaction on this Session.

        The target database(s) must support SQL SAVEPOINTs or a
        SQLAlchemy-supported vendor implementation of the idea.

        The nested transaction is a real transation, unlike a "subtransaction"
        which corresponds to multiple ``begin()`` calls.  The next
        ``rollback()`` or ``commit()`` call will operate upon this nested
        transaction.

        """
        return self.begin(nested=True)

    def rollback(self):
        """Rollback the current transaction in progress.

        If no transaction is in progress, this method is a pass-through.

        This method rolls back the current transaction or nested transaction
        regardless of subtransactions being in effect.  All subtransactions up
        to the first real transaction are closed.  Subtransactions occur when
        begin() is called multiple times.

        """
        if self.transaction is None:
            pass
        else:
            self.transaction.rollback()

    def commit(self):
        """Flush pending changes and commit the current transaction.

        If no transaction is in progress, this method raises an
        InvalidRequestError.

        If a subtransaction is in effect (which occurs when begin() is called
        multiple times), the subtransaction will be closed, and the next call
        to ``commit()`` will operate on the enclosing transaction.

        For a session configured with autocommit=False, a new transaction will
        be begun immediately after the commit, but note that the newly begun
        transaction does *not* use any connection resources until the first
        SQL is actually emitted.

        """
        if self.transaction is None:
            if not self.autocommit:
                self.begin()
            else:
                raise sa_exc.InvalidRequestError("No transaction is begun.")

        self.transaction.commit()

    def prepare(self):
        """Prepare the current transaction in progress for two phase commit.

        If no transaction is in progress, this method raises an
        InvalidRequestError.

        Only root transactions of two phase sessions can be prepared. If the
        current transaction is not such, an InvalidRequestError is raised.

        """
        if self.transaction is None:
            if not self.autocommit:
                self.begin()
            else:
                raise sa_exc.InvalidRequestError("No transaction is begun.")

        self.transaction.prepare()

    def connection(self, mapper=None, clause=None):
        """Return the active Connection.

        Retrieves the ``Connection`` managing the current transaction.  Any
        operations executed on the Connection will take place in the same
        transactional context as ``Session`` operations.

        For ``autocommit`` Sessions with no active manual transaction,
        ``connection()`` is a passthrough to ``contextual_connect()`` on the
        underlying engine.

        Ambiguity in multi-bind or unbound Sessions can be resolved through
        any of the optional keyword arguments.  See ``get_bind()`` for more
        information.

        mapper
          Optional, a ``mapper`` or mapped class

        clause
          Optional, any ``ClauseElement``

        """
        return self._connection_for_bind(self.get_bind(mapper, clause))

    def _connection_for_bind(self, engine, **kwargs):
        if self.transaction is not None:
            return self.transaction._connection_for_bind(engine)
        else:
            return engine.contextual_connect(**kwargs)

    def execute(self, clause, params=None, mapper=None, **kw):
        """Execute a clause within the current transaction.

        Returns a ``ResultProxy`` of execution results.  `autocommit` Sessions
        will create a transaction on the fly.

        Connection ambiguity in multi-bind or unbound Sessions will be
        resolved by inspecting the clause for binds.  The 'mapper' and
        'instance' keyword arguments may be used if this is insufficient, See
        ``get_bind()`` for more information.

        clause
            A ClauseElement (i.e. select(), text(), etc.) or
            string SQL statement to be executed

        params
            Optional, a dictionary of bind parameters.

        mapper
          Optional, a ``mapper`` or mapped class

        \**kw
          Additional keyword arguments are sent to :meth:`get_bind()`
          which locates a connectable to use for the execution.
          Subclasses of :class:`Session` may override this.
          
        """
        clause = expression._literal_as_text(clause)

        engine = self.get_bind(mapper, clause=clause, **kw)

        return self._connection_for_bind(engine, close_with_result=True).execute(
            clause, params or {})

    def scalar(self, clause, params=None, mapper=None, **kw):
        """Like execute() but return a scalar result."""
        
        return self.execute(clause, params=params, mapper=mapper, **kw).scalar()

    def close(self):
        """Close this Session.

        This clears all items and ends any transaction in progress.

        If this session were created with ``autocommit=False``, a new
        transaction is immediately begun.  Note that this new transaction does
        not use any connection resources until they are first needed.

        """
        self.expunge_all()
        if self.transaction is not None:
            for transaction in self.transaction._iterate_parents():
                transaction.close()

    @classmethod
    def close_all(cls):
        """Close *all* sessions in memory."""

        for sess in _sessions.values():
            sess.close()

    def expunge_all(self):
        """Remove all object instances from this ``Session``.

        This is equivalent to calling ``expunge(obj)`` on all objects in this
        ``Session``.

        """
        for state in self.identity_map.all_states() + list(self._new):
            state.detach()

        self.identity_map = self._identity_cls()
        self._new = {}
        self._deleted = {}

    # TODO: need much more test coverage for bind_mapper() and similar !
    # TODO: + crystalize + document resolution order vis. bind_mapper/bind_table

    def bind_mapper(self, mapper, bind):
        """Bind operations for a mapper to a Connectable.

        mapper
          A mapper instance or mapped class

        bind
          Any Connectable: a ``Engine`` or ``Connection``.

        All subsequent operations involving this mapper will use the given
        `bind`.

        """
        if isinstance(mapper, type):
            mapper = _class_mapper(mapper)

        self.__binds[mapper.base_mapper] = bind
        for t in mapper._all_tables:
            self.__binds[t] = bind

    def bind_table(self, table, bind):
        """Bind operations on a Table to a Connectable.

        table
          A ``Table`` instance

        bind
          Any Connectable: a ``Engine`` or ``Connection``.

        All subsequent operations involving this ``Table`` will use the
        given `bind`.

        """
        self.__binds[table] = bind

    def get_bind(self, mapper, clause=None):
        """Return an engine corresponding to the given arguments.

        All arguments are optional.

        mapper
          Optional, a ``Mapper`` or mapped class

        clause
          Optional, A ClauseElement (i.e. select(), text(), etc.)

        """
        if mapper is clause is None:
            if self.bind:
                return self.bind
            else:
                raise sa_exc.UnboundExecutionError(
                    "This session is not bound to a single Engine or "
                    "Connection, and no context was provided to locate "
                    "a binding.")

        c_mapper = mapper is not None and _class_to_mapper(mapper) or None
        
        # manually bound?
        if self.__binds:
            if c_mapper:
                if c_mapper.base_mapper in self.__binds:
                    return self.__binds[c_mapper.base_mapper]
                elif c_mapper.mapped_table in self.__binds:
                    return self.__binds[c_mapper.mapped_table]
            if clause is not None:
                for t in sql_util.find_tables(clause, include_crud=True):
                    if t in self.__binds:
                        return self.__binds[t]

        if self.bind:
            return self.bind

        if isinstance(clause, sql.expression.ClauseElement) and clause.bind:
            return clause.bind

        if c_mapper and c_mapper.mapped_table.bind:
            return c_mapper.mapped_table.bind

        context = []
        if mapper is not None:
            context.append('mapper %s' % c_mapper)
        if clause is not None:
            context.append('SQL expression')
        
        raise sa_exc.UnboundExecutionError(
            "Could not locate a bind configured on %s or this Session" % (
            ', '.join(context)))

    def query(self, *entities, **kwargs):
        """Return a new ``Query`` object corresponding to this ``Session``."""

        return self._query_cls(entities, self, **kwargs)

    def _autoflush(self):
        if self.autoflush and not self._flushing:
            self.flush()

    def _finalize_loaded(self, states):
        for state, dict_ in states.items():
            state.commit_all(dict_, self.identity_map)

    def refresh(self, instance, attribute_names=None, lockmode=None):
        """Refresh the attributes on the given instance.

        A query will be issued to the database and all attributes will be
        refreshed with their current database value.

        Lazy-loaded relational attributes will remain lazily loaded, so that
        the instance-wide refresh operation will be followed immediately by
        the lazy load of that attribute.

        Eagerly-loaded relational attributes will eagerly load within the
        single refresh operation.

        :param attribute_names: optional.  An iterable collection of
          string attribute names indicating a subset of attributes to 
          be refreshed.
        
        :param lockmode: Passed to the :class:`~sqlalchemy.orm.query.Query` 
          as used by :meth:`~sqlalchemy.orm.query.Query.with_lockmode`.
        
        """
        try:
            state = attributes.instance_state(instance)
        except exc.NO_STATE:
            raise exc.UnmappedInstanceError(instance)
        self._validate_persistent(state)
        if self.query(_object_mapper(instance))._get(
                state.key, refresh_state=state,
                lockmode=lockmode,
                only_load_props=attribute_names) is None:
            raise sa_exc.InvalidRequestError(
                "Could not refresh instance '%s'" %
                mapperutil.instance_str(instance))

    def expire_all(self):
        """Expires all persistent instances within this Session."""

        for state in self.identity_map.all_states():
            _expire_state(state, state.dict, None, instance_dict=self.identity_map)

    def expire(self, instance, attribute_names=None):
        """Expire the attributes on an instance.

        Marks the attributes of an instance as out of date.  When an expired
        attribute is next accessed, query will be issued to the database and
        the attributes will be refreshed with their current database value.
        ``expire()`` is a lazy variant of ``refresh()``.

        The ``attribute_names`` argument is an iterable collection
        of attribute names indicating a subset of attributes to be
        expired.

        """
        try:
            state = attributes.instance_state(instance)
        except exc.NO_STATE:
            raise exc.UnmappedInstanceError(instance)
        self._validate_persistent(state)
        if attribute_names:
            _expire_state(state, state.dict, 
                                attribute_names=attribute_names, instance_dict=self.identity_map)
        else:
            # pre-fetch the full cascade since the expire is going to
            # remove associations
            cascaded = list(_cascade_state_iterator('refresh-expire', state))
            _expire_state(state, state.dict, None, instance_dict=self.identity_map)
            for (state, m, o) in cascaded:
                _expire_state(state, state.dict, None, instance_dict=self.identity_map)

    def prune(self):
        """Remove unreferenced instances cached in the identity map.

        Note that this method is only meaningful if "weak_identity_map" is set
        to False.  The default weak identity map is self-pruning.

        Removes any object in this Session's identity map that is not
        referenced in user code, modified, new or scheduled for deletion.
        Returns the number of objects pruned.

        """
        return self.identity_map.prune()

    def expunge(self, instance):
        """Remove the `instance` from this ``Session``.

        This will free all internal references to the instance.  Cascading
        will be applied according to the *expunge* cascade rule.

        """
        try:
            state = attributes.instance_state(instance)
        except exc.NO_STATE:
            raise exc.UnmappedInstanceError(instance)
        if state.session_id is not self.hash_key:
            raise sa_exc.InvalidRequestError(
                "Instance %s is not present in this Session" %
                mapperutil.state_str(state))
        for s, m, o in [(state, None, None)] + list(_cascade_state_iterator('expunge', state)):
            self._expunge_state(s)

    def _expunge_state(self, state):
        if state in self._new:
            self._new.pop(state)
            state.detach()
        elif self.identity_map.contains_state(state):
            self.identity_map.discard(state)
            self._deleted.pop(state, None)
            state.detach()

    def _register_newly_persistent(self, state):
        mapper = _state_mapper(state)

        # prevent against last minute dereferences of the object
        obj = state.obj()
        if obj is not None:

            instance_key = mapper._identity_key_from_state(state)

            if state.key is None:
                state.key = instance_key
            elif state.key != instance_key:
                # primary key switch.
                # use discard() in case another state has already replaced this
                # one in the identity map (see test/orm/test_naturalpks.py ReversePKsTest)
                self.identity_map.discard(state)
                state.key = instance_key
            
            self.identity_map.replace(state)
            state.commit_all(state.dict, self.identity_map)
            
        # remove from new last, might be the last strong ref
        if state in self._new:
            if self._enable_transaction_accounting and self.transaction:
                self.transaction._new[state] = True
            self._new.pop(state)

    def _remove_newly_deleted(self, state):
        if self._enable_transaction_accounting and self.transaction:
            self.transaction._deleted[state] = True

        self.identity_map.discard(state)
        self._deleted.pop(state, None)

    def _save_without_cascade(self, instance):
        """Used by scoping.py to save on init without cascade."""

        state = _state_for_unsaved_instance(instance, create=True)
        self._save_impl(state)

    def add(self, instance):
        """Place an object in the ``Session``.

        Its state will be persisted to the database on the next flush
        operation.

        Repeated calls to ``add()`` will be ignored. The opposite of ``add()``
        is ``expunge()``.

        """
        state = _state_for_unknown_persistence_instance(instance)
        self._save_or_update_state(state)

    def add_all(self, instances):
        """Add the given collection of instances to this ``Session``."""

        for instance in instances:
            self.add(instance)

    def _save_or_update_state(self, state):
        self._save_or_update_impl(state)
        self._cascade_save_or_update(state)

    def _cascade_save_or_update(self, state):
        for state, mapper in _cascade_unknown_state_iterator(
                                    'save-update', state, halt_on=self.__contains__):
            self._save_or_update_impl(state)

    def delete(self, instance):
        """Mark an instance as deleted.

        The database delete operation occurs upon ``flush()``.

        """
        try:
            state = attributes.instance_state(instance)
        except exc.NO_STATE:
            raise exc.UnmappedInstanceError(instance)

        if state.key is None:
            raise sa_exc.InvalidRequestError(
                "Instance '%s' is not persisted" %
                mapperutil.state_str(state))

        if state in self._deleted:
            return
            
        # ensure object is attached to allow the 
        # cascade operation to load deferred attributes
        # and collections
        self._attach(state)

        # grab the cascades before adding the item to the deleted list
        # so that autoflush does not delete the item
        cascade_states = list(_cascade_state_iterator('delete', state))

        self._deleted[state] = state.obj()
        self.identity_map.add(state)

        for state, m, o in cascade_states:
            self._delete_impl(state)

    def merge(self, instance, load=True, **kw):
        """Copy the state an instance onto the persistent instance with the same identifier.

        If there is no persistent instance currently associated with the
        session, it will be loaded.  Return the persistent instance. If the
        given instance is unsaved, save a copy of and return it as a newly
        persistent instance. The given instance does not become associated
        with the session.

        This operation cascades to associated instances if the association is
        mapped with ``cascade="merge"``.

        """
        if 'dont_load' in kw:
            load = not kw['dont_load']
            util.warn_deprecated("dont_load=True has been renamed to load=False.")
        
        _recursive = {}
        
        if load:
            # flush current contents if we expect to load data
            self._autoflush()
            
        _object_mapper(instance) # verify mapped
        autoflush = self.autoflush
        try:
            self.autoflush = False
            return self._merge(
                            attributes.instance_state(instance), 
                            attributes.instance_dict(instance), 
                            load=load, _recursive=_recursive)
        finally:
            self.autoflush = autoflush
        
    def _merge(self, state, state_dict, load=True, _recursive=None):
        mapper = _state_mapper(state)
        if state in _recursive:
            return _recursive[state]

        new_instance = False
        key = state.key
        
        if key is None:
            if not load:
                raise sa_exc.InvalidRequestError(
                    "merge() with load=False option does not support "
                    "objects transient (i.e. unpersisted) objects.  flush() "
                    "all changes on mapped instances before merging with "
                    "load=False.")
            key = mapper._identity_key_from_state(state)

        if key in self.identity_map:
            merged = self.identity_map[key]
            
        elif not load:
            if state.modified:
                raise sa_exc.InvalidRequestError(
                    "merge() with load=False option does not support "
                    "objects marked as 'dirty'.  flush() all changes on "
                    "mapped instances before merging with load=False.")
            merged = mapper.class_manager.new_instance()
            merged_state = attributes.instance_state(merged)
            merged_state.key = key
            self._update_impl(merged_state)
            new_instance = True
        
        elif not _none_set.issubset(key[1]) or \
                    (mapper.allow_partial_pks and 
                    not _none_set.issuperset(key[1])):
            merged = self.query(mapper.class_).get(key[1])
        else:
            merged = None
            
        if merged is None:
            merged = mapper.class_manager.new_instance()
            merged_state = attributes.instance_state(merged)
            merged_dict = attributes.instance_dict(merged)
            new_instance = True
            self._save_or_update_state(merged_state)
        else:
            merged_state = attributes.instance_state(merged)
            merged_dict = attributes.instance_dict(merged)
            
        _recursive[state] = merged

        # check that we didn't just pull the exact same
        # state out.   
        if state is not merged_state:
            merged_state.load_path = state.load_path
            merged_state.load_options = state.load_options
            
            for prop in mapper.iterate_properties:
                prop.merge(self, state, state_dict, merged_state, merged_dict, load, _recursive)

        if not load:
            # remove any history
            merged_state.commit_all(merged_dict, self.identity_map)  

        if new_instance:
            merged_state._run_on_load(merged)
        return merged

    @classmethod
    def identity_key(cls, *args, **kwargs):
        return mapperutil.identity_key(*args, **kwargs)

    @classmethod
    def object_session(cls, instance):
        """Return the ``Session`` to which an object belongs."""

        return object_session(instance)

    def _validate_persistent(self, state):
        if not self.identity_map.contains_state(state):
            raise sa_exc.InvalidRequestError(
                "Instance '%s' is not persistent within this Session" %
                mapperutil.state_str(state))

    def _save_impl(self, state):
        if state.key is not None:
            raise sa_exc.InvalidRequestError(
                "Object '%s' already has an identity - it can't be registered "
                "as pending" % mapperutil.state_str(state))
                
        self._attach(state)
        if state not in self._new:
            self._new[state] = state.obj()
            state.insert_order = len(self._new)

    def _update_impl(self, state):
        if (self.identity_map.contains_state(state) and
            state not in self._deleted):
            return
            
        if state.key is None:
            raise sa_exc.InvalidRequestError(
                "Instance '%s' is not persisted" %
                mapperutil.state_str(state))

        self._attach(state)
        self._deleted.pop(state, None)
        self.identity_map.add(state)

    def _save_or_update_impl(self, state):
        if state.key is None:
            self._save_impl(state)
        else:
            self._update_impl(state)

    def _delete_impl(self, state):
        if state in self._deleted:
            return

        if state.key is None:
            return
                    
        self._attach(state)
        self._deleted[state] = state.obj()
        self.identity_map.add(state)
    
    def _attach(self, state):
        if state.key and \
            state.key in self.identity_map and \
            not self.identity_map.contains_state(state):
            raise sa_exc.InvalidRequestError(
                "Can't attach instance %s; another instance with key %s is already present in this session." % 
                    (mapperutil.state_str(state), state.key)
                )
                
        if state.session_id and state.session_id is not self.hash_key:
            raise sa_exc.InvalidRequestError(
                "Object '%s' is already attached to session '%s' "
                "(this is '%s')" % (mapperutil.state_str(state),
                                    state.session_id, self.hash_key))
                                    
        if state.session_id != self.hash_key:
            state.session_id = self.hash_key
            for ext in self.extensions:
                ext.after_attach(self, state.obj())

    def __contains__(self, instance):
        """Return True if the instance is associated with this session.

        The instance may be pending or persistent within the Session for a
        result of True.

        """
        try:
            state = attributes.instance_state(instance)
        except exc.NO_STATE:
            raise exc.UnmappedInstanceError(instance)
        return self._contains_state(state)

    def __iter__(self):
        """Iterate over all pending or persistent instances within this Session."""

        return iter(list(self._new.values()) + self.identity_map.values())

    def _contains_state(self, state):
        return state in self._new or self.identity_map.contains_state(state)

    def flush(self, objects=None):
        """Flush all the object changes to the database.

        Writes out all pending object creations, deletions and modifications
        to the database as INSERTs, DELETEs, UPDATEs, etc.  Operations are
        automatically ordered by the Session's unit of work dependency
        solver..

        Database operations will be issued in the current transactional
        context and do not affect the state of the transaction.  You may
        flush() as often as you like within a transaction to move changes from
        Python to the database's transaction buffer.

        For ``autocommit`` Sessions with no active manual transaction, flush()
        will create a transaction on the fly that surrounds the entire set of
        operations int the flush.

        objects
          Optional; a list or tuple collection.  Restricts the flush operation
          to only these objects, rather than all pending changes.
          Deprecated - this flag prevents the session from properly maintaining
          accounting among inter-object relations and can cause invalid results.

        """

        if objects:
            util.warn_deprecated(
                "The 'objects' argument to session.flush() is deprecated; "
                "Please do not add objects to the session which should not yet be persisted.")
        
        if self._flushing:
            raise sa_exc.InvalidRequestError("Session is already flushing")
            
        try:
            self._flushing = True
            self._flush(objects)
        finally:
            self._flushing = False
            
    def _flush(self, objects=None):
        if (not self.identity_map.check_modified() and
            not self._deleted and not self._new):
            return

        dirty = self._dirty_states
        if not dirty and not self._deleted and not self._new:
            self.identity_map._modified.clear()
            return

        flush_context = UOWTransaction(self)

        if self.extensions:
            for ext in self.extensions:
                ext.before_flush(self, flush_context, objects)
            dirty = self._dirty_states
            
        deleted = set(self._deleted)
        new = set(self._new)

        dirty = set(dirty).difference(deleted)

        # create the set of all objects we want to operate upon
        if objects:
            # specific list passed in
            objset = set()
            for o in objects:
                try:
                    state = attributes.instance_state(o)
                except exc.NO_STATE:
                    raise exc.UnmappedInstanceError(o)
                objset.add(state)
        else:
            objset = None

        # store objects whose fate has been decided
        processed = set()

        # put all saves/updates into the flush context.  detect top-level
        # orphans and throw them into deleted.
        if objset:
            proc = new.union(dirty).intersection(objset).difference(deleted)
        else:
            proc = new.union(dirty).difference(deleted)
            
        for state in proc:
            is_orphan = _state_mapper(state)._is_orphan(state)
            if is_orphan and not _state_has_identity(state):
                path = ", nor ".join(
                    ["any parent '%s' instance "
                     "via that classes' '%s' attribute" %
                     (cls.__name__, key)
                     for (key, cls) in chain(*(m.delete_orphans for m in _state_mapper(state).iterate_to_root()))])
                raise exc.FlushError(
                    "Instance %s is an unsaved, pending instance and is an "
                    "orphan (is not attached to %s)" % (
                    mapperutil.state_str(state), path))
            flush_context.register_object(state, isdelete=is_orphan)
            processed.add(state)

        # put all remaining deletes into the flush context.
        if objset:
            proc = deleted.intersection(objset).difference(processed)
        else:
            proc = deleted.difference(processed)
        for state in proc:
            flush_context.register_object(state, isdelete=True)

        if len(flush_context.tasks) == 0:
            return

        flush_context.transaction = transaction = self.begin(
            subtransactions=True)
        try:
            flush_context.execute()

            for ext in self.extensions:
                ext.after_flush(self, flush_context)
            transaction.commit()
        except:
            transaction.rollback()
            raise
        
        flush_context.finalize_flush_changes()

        # useful assertions:
        #if not objects:
        #    assert not self.identity_map._modified
        #else:
        #    assert self.identity_map._modified == self.identity_map._modified.difference(objects)
        #self.identity_map._modified.clear()
        
        for ext in self.extensions:
            ext.after_flush_postexec(self, flush_context)

    def is_modified(self, instance, include_collections=True, passive=False):
        """Return True if instance has modified attributes.

        This method retrieves a history instance for each instrumented
        attribute on the instance and performs a comparison of the current
        value to its previously committed value.  Note that instances present
        in the 'dirty' collection may result in a value of ``False`` when
        tested with this method.

        `include_collections` indicates if multivalued collections should be
        included in the operation.  Setting this to False is a way to detect
        only local-column based properties (i.e. scalar columns or many-to-one
        foreign keys) that would result in an UPDATE for this instance upon
        flush.

        The `passive` flag indicates if unloaded attributes and collections
        should not be loaded in the course of performing this test.

        """
        try:
            state = attributes.instance_state(instance)
        except exc.NO_STATE:
            raise exc.UnmappedInstanceError(instance)
        dict_ = state.dict
        for attr in state.manager.attributes:
            if \
                (
                    not include_collections and 
                    hasattr(attr.impl, 'get_collection')
                ) or not hasattr(attr.impl, 'get_history'):
                continue
                
            (added, unchanged, deleted) = \
                    attr.impl.get_history(state, dict_, passive=passive)
                                            
            if added or deleted:
                return True
        return False

    @property
    def is_active(self):
        """True if this Session has an active transaction."""

        return self.transaction and self.transaction.is_active

    @property
    def _dirty_states(self):
        """The set of all persistent states considered dirty.

        This method returns all states that were modified including
        those that were possibly deleted.

        """
        return self.identity_map._dirty_states()

    @property
    def dirty(self):
        """The set of all persistent instances considered dirty.

        Instances are considered dirty when they were modified but not
        deleted.

        Note that this 'dirty' calculation is 'optimistic'; most
        attribute-setting or collection modification operations will
        mark an instance as 'dirty' and place it in this set, even if
        there is no net change to the attribute's value.  At flush
        time, the value of each attribute is compared to its
        previously saved value, and if there's no net change, no SQL
        operation will occur (this is a more expensive operation so
        it's only done at flush time).

        To check if an instance has actionable net changes to its
        attributes, use the is_modified() method.

        """
        return util.IdentitySet(
            [state.obj()
             for state in self._dirty_states
             if state not in self._deleted])

    @property
    def deleted(self):
        "The set of all instances marked as 'deleted' within this ``Session``"

        return util.IdentitySet(self._deleted.values())

    @property
    def new(self):
        "The set of all instances marked as 'new' within this ``Session``."

        return util.IdentitySet(self._new.values())

_expire_state = state.InstanceState.expire_attributes
    
UOWEventHandler = unitofwork.UOWEventHandler

_sessions = weakref.WeakValueDictionary()

def _cascade_state_iterator(cascade, state, **kwargs):
    mapper = _state_mapper(state)
    # yield the state, object, mapper.  yielding the object
    # allows the iterator's results to be held in a list without
    # states being garbage collected
    for (o, m) in mapper.cascade_iterator(cascade, state, **kwargs):
        yield attributes.instance_state(o), o, m

def _cascade_unknown_state_iterator(cascade, state, **kwargs):
    mapper = _state_mapper(state)
    for (o, m) in mapper.cascade_iterator(cascade, state, **kwargs):
        yield _state_for_unknown_persistence_instance(o), m

def _state_for_unsaved_instance(instance, create=False):
    try:
        state = attributes.instance_state(instance)
    except AttributeError:
        raise exc.UnmappedInstanceError(instance)
    if state:
        if state.key is not None:
            raise sa_exc.InvalidRequestError(
                "Instance '%s' is already persistent" %
                mapperutil.state_str(state))
    elif create:
        manager = attributes.manager_of_class(instance.__class__)
        if manager is None:
            raise exc.UnmappedInstanceError(instance)
        state = manager.setup_instance(instance)
    else:
        raise exc.UnmappedInstanceError(instance)

    return state

def _state_for_unknown_persistence_instance(instance):
    try:
        state = attributes.instance_state(instance)
    except exc.NO_STATE:
        raise exc.UnmappedInstanceError(instance)

    return state

def make_transient(instance):
    """Make the given instance 'transient'.
    
    This will remove its association with any 
    session and additionally will remove its "identity key",
    such that it's as though the object were newly constructed,
    except retaining its values.
    
    """
    state = attributes.instance_state(instance)
    s = _state_session(state)
    if s:
        s._expunge_state(state)
    del state.key
    
    
def object_session(instance):
    """Return the ``Session`` to which instance belongs, or None."""

    return _state_session(attributes.instance_state(instance))

def _state_session(state):
    if state.session_id:
        try:
            return _sessions[state.session_id]
        except KeyError:
            pass
    return None

# Lazy initialization to avoid circular imports
unitofwork.object_session = object_session
unitofwork._state_session = _state_session
from sqlalchemy.orm import mapper
mapper._expire_state = _expire_state
mapper._state_session = _state_session