[Sqlalchemy-tickets] Issue #3254: Join should be able to follow relationships implicitly (zzzeek/sqlalchemy)

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

New issue 3254: Join should be able to follow relationships implicitly
https://bitbucket.org/zzzeek/sqlalchemy/issue/3254/join-should-be-able-to-follow

nickretallack:

It's my opinion that this should work:

```
#!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

Model = declarative_base()

class Parent(Model):
	__tablename__ = 'parent'
	id = Column(Integer, primary_key=True)
	name = Column(String, nullable=False)

class Child(Model):
	__tablename__ = 'child'
	id = Column(Integer, primary_key=True)
	name = Column(String, nullable=False)
	parent_id = Column(Integer, nullable=False)
	parent = relationship('Parent', primaryjoin='foreign(Child.parent_id) == remote(Parent.id)')

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

if __name__ == "__main__":
	Model.metadata.create_all(engine)
	session = Session()

	# setup
	parent = Parent(name="fred")
	child = Child(name="bob", parent=parent)

	session.add_all([parent, child])
	session.commit()

	# payoff
	print session.query(Parent).join(Child).all()
```

It works if parent_id has a ForeignKeyConstraint.  It also works if I change it to ```print session.query(Parent).join(Child.parent).all()```.  But I think it should work without either of these changes.

It would be very helpful in an application I'm working on now, which mixes queries very dynamically.  I suppose I could work around it by searching my models for relationships to follow manually, or adding foreign key constraints to the model that do not actually exist in the database.  I just wish it worked without having to do these things.