diff --git a/dibbler/models/Transaction.py b/dibbler/models/Transaction.py
index c59929c..e446a38 100644
--- a/dibbler/models/Transaction.py
+++ b/dibbler/models/Transaction.py
@@ -56,8 +56,6 @@ _EXPECTED_FIELDS: dict[TransactionType, set[str]] = {
     TransactionType.ADJUST_INTEREST: {"interest_rate_percent"},
     TransactionType.ADJUST_PENALTY: {"penalty_multiplier_percent", "penalty_threshold"},
     TransactionType.ADJUST_STOCK: {"product_count", "product_id"},
-    # TODO: remove amount from BUY_PRODUCT
-    #       this requires modifications to user credit calculations
     TransactionType.BUY_PRODUCT: {"product_count", "product_id"},
     TransactionType.TRANSFER: {"amount", "transfer_user_id"},
 }
@@ -91,6 +89,10 @@ class Transaction(Base):
             _transaction_type_field_constraints(transaction_type, expected_fields)
             for transaction_type, expected_fields in _EXPECTED_FIELDS.items()
         ],
+        CheckConstraint(
+            f"type <> '{TransactionType.TRANSFER}' OR user_id <> transfer_user_id",
+            name="trx_type_transfer_no_self_transfers",
+        ),
         # Speed up product count calculation
         Index("product_user_time", "product_id", "user_id", "time"),
         # Speed up product owner calculation
diff --git a/tests/models/test_transaction.py b/tests/models/test_transaction.py
index a7c4eb2..2b2c785 100644
--- a/tests/models/test_transaction.py
+++ b/tests/models/test_transaction.py
@@ -40,3 +40,81 @@ def test_transaction_no_duplicate_timestamps(sql_session: Session):
 
     with pytest.raises(IntegrityError):
         sql_session.commit()
+
+
+def test_user_not_allowed_to_transfer_to_self(sql_session: Session) -> None:
+    user, _ = insert_test_data(sql_session)
+
+    transaction = Transaction.transfer(
+        time=datetime(2023, 10, 1, 12, 0, 0),
+        user_id=user.id,
+        transfer_user_id=user.id,
+        amount=50,
+    )
+
+    sql_session.add(transaction)
+
+    with pytest.raises(IntegrityError):
+        sql_session.commit()
+
+
+def test_product_foreign_key_constraint(sql_session: Session) -> None:
+    user, product = insert_test_data(sql_session)
+
+    transaction = Transaction.add_product(
+        time=datetime(2023, 10, 1, 12, 0, 0),
+        user_id=user.id,
+        product_id=product.id,
+        amount=27,
+        per_product=27,
+        product_count=1,
+    )
+
+    sql_session.add(transaction)
+    sql_session.commit()
+
+    # Attempt to add a transaction with a non-existent product
+    invalid_transaction = Transaction.add_product(
+        time=datetime(2023, 10, 1, 12, 0, 1),
+        user_id=user.id,
+        product_id=9999,  # Non-existent product ID
+        amount=27,
+        per_product=27,
+        product_count=1,
+    )
+
+    sql_session.add(invalid_transaction)
+
+    with pytest.raises(IntegrityError):
+        sql_session.commit()
+
+
+def test_user_foreign_key_constraint(sql_session: Session) -> None:
+    user, product = insert_test_data(sql_session)
+
+    transaction = Transaction.add_product(
+        time=datetime(2023, 10, 1, 12, 0, 0),
+        user_id=user.id,
+        product_id=product.id,
+        amount=27,
+        per_product=27,
+        product_count=1,
+    )
+
+    sql_session.add(transaction)
+    sql_session.commit()
+
+    # Attempt to add a transaction with a non-existent user
+    invalid_transaction = Transaction.add_product(
+        time=datetime(2023, 10, 1, 12, 0, 1),
+        user_id=9999,  # Non-existent user ID
+        product_id=product.id,
+        amount=27,
+        per_product=27,
+        product_count=1,
+    )
+
+    sql_session.add(invalid_transaction)
+
+    with pytest.raises(IntegrityError):
+        sql_session.commit()
diff --git a/tests/queries/test_transfer_balance.py b/tests/queries/test_transfer_balance.py
deleted file mode 100644
index 389e973..0000000
--- a/tests/queries/test_transfer_balance.py
+++ /dev/null
@@ -1,12 +0,0 @@
-from sqlalchemy.orm import Session
-
-
-def test_user_not_allowed_to_transfer_to_self(sql_session: Session) -> None:
-    raise NotImplementedError("This test is not implemented yet.")
-#     insert_test_data(sql_session)
-#     ...
-
-# user1 = sql_session.scalars(select(User).where(User.name == "Test User 1")).one()
-
-# with pytest.raises(ValueError, match="Cannot transfer to self"):
-#     user1.transfer(sql_session, user1, 10)  # Attempting to transfer to self