Write a set of queries to go along with the event sourcing model

tests/queries/test_adjust_interest.py

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+from datetime import datetime, timedelta
+
+import pytest
+from sqlalchemy.orm import Session
+
+from dibbler.models import Transaction, User
+from dibbler.queries import adjust_interest, current_interest
+
+
+def insert_test_data(sql_session: Session) -> User:
+    user = User("Test User")
+    sql_session.add(user)
+    sql_session.commit()
+
+    return user
+
+
+def test_adjust_interest_unitialized_user(sql_session: Session) -> None:
+    user = User("Uninitialized User")
+
+    with pytest.raises(ValueError, match="User must be persisted in the database."):
+        adjust_interest(
+            sql_session,
+            user=user,
+            new_interest=4,
+            message="Attempting to adjust interest for uninitialized user",
+        )
+
+
+def test_adjust_interest_no_history(sql_session: Session) -> None:
+    user = insert_test_data(sql_session)
+
+    adjust_interest(
+        sql_session,
+        user=user,
+        new_interest=3,
+        message="Setting initial interest rate",
+    )
+    sql_session.commit()
+
+    current_interest_rate = current_interest(sql_session)
+
+    assert current_interest_rate == 3
+
+
+def test_adjust_interest_existing_history(sql_session: Session) -> None:
+    user = insert_test_data(sql_session)
+
+    transactions = [
+        Transaction.adjust_interest(
+            time=datetime(2023, 10, 1, 9, 0, 0),
+            user_id=user.id,
+            interest_rate_percent=5,
+            message="Initial interest rate",
+        ),
+    ]
+    sql_session.add_all(transactions)
+    sql_session.commit()
+
+    current_interest_rate = current_interest(sql_session)
+    assert current_interest_rate == 5
+
+    adjust_interest(
+        sql_session,
+        user=user,
+        new_interest=2,
+        message="Adjusting interest rate",
+        time=transactions[-1].time + timedelta(days=1),
+    )
+    sql_session.commit()
+
+    current_interest_rate = current_interest(sql_session)
+    assert current_interest_rate == 2
+
+
+def test_adjust_interest_negative_failure(sql_session: Session) -> None:
+    user = insert_test_data(sql_session)
+
+    with pytest.raises(ValueError, match="Interest rate cannot be negative"):
+        adjust_interest(
+            sql_session,
+            user=user,
+            new_interest=-1,
+            message="Attempting to set negative interest rate",
+        )