gistfile1.py

   pool_account_result_mapper = orm.mapper(
        PoolAccountResult,
        table['pool_invite_result'],
        properties = {
            'result' : orm.relationship(Result,
                                        backref='pool_account_result_assoc'),

            'pool' : orm.relationship(
                Pool,
                primaryjoin=sql.and_(
                    table['pool_invite_result'].c.pool_invite_pool_id ==
                    table['pool_invite'].c.pool_id,

                    table['pool_invite'].c.pool_id == table['pool'].c.id
                ),
                uselist = False,
                innerjoin = True
            ),

            'invited_user' : orm.relationship(
                Human,
                primaryjoin=sql.and_(
                    table['pool_invite_result'].c.pool_invite_account_id ==
                    table['pool_invite'].c.account_id,

                    table['pool_invite'].c.account_id == table['account'].c.id
                ),
                uselist = False,
                innerjoin = True
            )
        })
  
# Fail with:
# sqlalchemy.exc.ArgumentError: Could not locate any simple equality expressions involving locally mapped foreign key columns for primary join condition 'pool_invite_result.pool_invite_pool_id = pool_invite.pool_id AND pool_invite.pool_id = pool.id' on relationship PoolAccountResult.pool.  Ensure that referencing columns are associated with a ForeignKey or ForeignKeyConstraint, or are annotated in the join condition with the foreign() annotation. To allow comparison operators other than '==', the relationship can be marked as viewonly=True.