[Sqlalchemy-tickets] Issue #4365: dont assume _from_obj[0] when joining without a clear "left" side (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 4365: dont assume _from_obj[0] when joining without a clear "left" side
https://bitbucket.org/zzzeek/sqlalchemy/issues/4365/dont-assume-_from_obj-0-when-joining

Michael Bayer:

this would allow us to fix the hack in #4363


```
#!python

    def test_join_entity_from_multiple_from_clause(self):
        """test adding joins onto multiple FROM clauses"""

        User, Address = (self.classes.User,
                         self.classes.Address)
        sess = create_session()

        q = sess.query(Address, User).join(Address.dingaling).\
            join(User.orders)

        a1 = aliased(Address)

        assert_raises_message(
            sa.exc.InvalidRequestError,
            "Can't determine which FROM clause to join from, there are "
            "multiple FROMS which can join to this entity",
            q.join, a1
        )

    def test_join_entity_w_onclause_from_multiple_from_clause(self):
        """test adding joins onto multiple FROM clauses"""

        User, Order, Address = (self.classes.User,
                                self.classes.Order,
                                self.classes.Address)
        sess = create_session()

        q = sess.query(Address, User).join(Address.dingaling).\
            join(User.orders)

        a1 = aliased(Address)

        q = q.join(a1, Order.address_id == a1.id)
        self.assert_compile(
            q,
            "< query works >"
        )

```