[Sqlalchemy-tickets] Issue #3230: Overlapping Composite Keys confuse the Dependency Rule (zzzeek/sqlalchemy)

[nickretallack <iss...@bi...>]

New issue 3230: Overlapping Composite Keys confuse the Dependency Rule
https://bitbucket.org/zzzeek/sqlalchemy/issue/3230/overlapping-composite-keys-confuse-the

nickretallack:

I see there was [already an issue about overlapping composite keys in the past](https://bitbucket.org/zzzeek/sqlalchemy/issue/2965/foreign_keys-logic-with-overlapping), but maybe mine's different, as this issue still exists in 0.9.8.  The issue occurs when you try to un-relate some records with overlapping keys.

Here's my example:

```
#!python

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, ForeignKey, Integer, String, ForeignKeyConstraint
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, relationship

Base = declarative_base()

# Lets simulate a multi-tenant database system that doesn't use multiple schemas
class Tenant(Base):
	__tablename__ = 'tenant'
	id = Column(Integer, primary_key=True)
	name = Column(String)

# Something to be related to
class Friend(Base):
	__tablename__ = 'friend'
	id = Column(Integer, primary_key=True)
	tenant_id = Column(ForeignKey('tenant.id'), primary_key=True)
	name = Column(String)
	tenant = relationship(Tenant)

# Our model
class Person(Base):
	__tablename__ = 'person'
	id = Column(Integer, primary_key=True)
	tenant_id = Column(ForeignKey('tenant.id'), primary_key=True)
	friend_id = Column(Integer)
	name = Column(String)

	__table_args__ = (
		# friend relationship
		ForeignKeyConstraint(
			(friend_id, tenant_id),
			('friend.id','friend.tenant_id'),
		),
	)

	tenant = relationship(Tenant)
	friend = relationship(Friend)

engine = create_engine('sqlite:///:memory:', echo=True)
Session = sessionmaker(bind=engine)

def run():
	# Basic Setup
	Base.metadata.create_all(engine)
	session = Session()

	# Make a person with a friend
	tenant = Tenant(name="testing", id=1)
	person = Person(name="Bob", tenant=tenant, id=1)
	friend = Friend(name="George", tenant=tenant, id=1)
	person.friend = friend
	session.add_all([tenant, person, friend])
	session.commit()

	# Take away the friend
	person = session.query(Person).first()
	person.friend = None
	session.commit()
	# it tries to take away the tenant_id too, thus breaking the primary key

if __name__ == '__main__':
	run()
```

When I run it, I get this:

```
AssertionError: Dependency rule tried to blank-out primary key column 'person.tenant_id' on instance '<Person at 0x105c05310>'
```

It shouldn't be blanking out the tenant_id, since that id still references the tenant.  The relationship needs to know that, despite joining on a composite key here, only one of those columns is specific to this relationship.  The other is shared with many other relationships.