How do I do a join without a real foreign key constraint?

joins.py

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()

output.txt

2014-11-20 17:24:21,129 INFO sqlalchemy.engine.base.Engine SELECT CAST('test plain returns' AS VARCHAR(60)) AS anon_1
2014-11-20 17:24:21,129 INFO sqlalchemy.engine.base.Engine ()
2014-11-20 17:24:21,130 INFO sqlalchemy.engine.base.Engine SELECT CAST('test unicode returns' AS VARCHAR(60)) AS anon_1
2014-11-20 17:24:21,130 INFO sqlalchemy.engine.base.Engine ()
2014-11-20 17:24:21,130 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("parent")
2014-11-20 17:24:21,131 INFO sqlalchemy.engine.base.Engine ()
2014-11-20 17:24:21,131 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("child")
2014-11-20 17:24:21,131 INFO sqlalchemy.engine.base.Engine ()
2014-11-20 17:24:21,131 INFO sqlalchemy.engine.base.Engine
CREATE TABLE parent (
	id INTEGER NOT NULL,
	name VARCHAR NOT NULL,
	PRIMARY KEY (id)
)


2014-11-20 17:24:21,131 INFO sqlalchemy.engine.base.Engine ()
2014-11-20 17:24:21,132 INFO sqlalchemy.engine.base.Engine COMMIT
2014-11-20 17:24:21,132 INFO sqlalchemy.engine.base.Engine
CREATE TABLE child (
	id INTEGER NOT NULL,
	name VARCHAR NOT NULL,
	parent_id INTEGER NOT NULL,
	PRIMARY KEY (id)
)


2014-11-20 17:24:21,132 INFO sqlalchemy.engine.base.Engine ()
2014-11-20 17:24:21,132 INFO sqlalchemy.engine.base.Engine COMMIT
Traceback (most recent call last):
  File "joins.py", line 35, in <module>
    print session.query(Parent).join(Child).all()
  File "/Users/nickretallack/virtualenvs/newflask/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 1724, in join
    from_joinpoint=from_joinpoint)
  File "<string>", line 2, in _join
  File "/Users/nickretallack/virtualenvs/newflask/lib/python2.7/site-packages/sqlalchemy/orm/base.py", line 191, in generate
    fn(self, *args[1:], **kw)
  File "/Users/nickretallack/virtualenvs/newflask/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 1858, in _join
    outerjoin, create_aliases, prop)
  File "/Users/nickretallack/virtualenvs/newflask/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 1928, in _join_left_to_right
    self._join_to_left(l_info, left, right, onclause, outerjoin)
  File "/Users/nickretallack/virtualenvs/newflask/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2056, in _join_to_left
    "Tried joining to %s, but got: %s" % (right, ae))
sqlalchemy.exc.InvalidRequestError: Could not find a FROM clause to join from.  Tried joining to <class '__main__.Child'>, but got: Can't find any foreign key relationships between 'parent' and 'child'.

@xuyang2 , thank you！