[Sqlalchemy-tickets] Issue #4328: TypeError when using bakery-cached ShardedQuery for Query (zzzeek/sqlalchemy)

[Carson I. <iss...@bi...>]

New issue 4328: TypeError when using bakery-cached ShardedQuery for Query
https://bitbucket.org/zzzeek/sqlalchemy/issues/4328/typeerror-when-using-bakery-cached

Carson Ip:

Create a bakery, bake a query using ShardedSession such that the cached query is of type ShardedQuery. Then query using a normal Session. Raises TypeError on unexpected kwarg "shard_id". The order of querying using ShardedSession and Session matters.

Stack trace:
```
Traceback (most recent call last):
  File "sqlalchemy/test/ext/test_horizontal_shard.py", line 330, in test_baked_mix
    t = get_tokyo(sess2)
  File "sqlalchemy/test/ext/test_horizontal_shard.py", line 318, in get_tokyo
    t = bq(sess).get(tokyo.id)
  File "sqlalchemy/test/../lib/sqlalchemy/ext/baked.py", line 458, in get
    return query._get_impl(ident, self._load_on_pk_identity)
  File "sqlalchemy/test/../lib/sqlalchemy/orm/query.py", line 1004, in _get_impl
    return db_load_fn(self, primary_key_identity)
  File "sqlalchemy/test/../lib/sqlalchemy/ext/baked.py", line 510, in _load_on_pk_identity
    result = list(bq.for_session(self.session).params(**params))
  File "sqlalchemy/test/../lib/sqlalchemy/ext/baked.py", line 355, in __iter__
    return q._execute_and_instances(context)
  File "sqlalchemy/test/../lib/sqlalchemy/ext/horizontal_shard.py", line 61, in _execute_and_instances
    partial.extend(iter_for_shard(shard_id))
  File "sqlalchemy/test/../lib/sqlalchemy/ext/horizontal_shard.py", line 49, in iter_for_shard
    shard_id=shard_id).execute(
  File "sqlalchemy/test/../lib/sqlalchemy/orm/query.py", line 2998, in _connection_from_session
    conn = self.session.connection(**kw)
  File "sqlalchemy/test/../lib/sqlalchemy/orm/session.py", line 1031, in connection
    bind = self.get_bind(mapper, clause=clause, **kw)
TypeError: get_bind() got an unexpected keyword argument 'shard_id'
```

To reproduce, add this test case under test_horizontal_shard: (I know it is a bad place to put the test case, but it is more convenient to reproduce here)
```
    def test_baked_mix(self):
        sess = self._fixture_data()

        tokyo = sess.query(WeatherLocation).filter_by(city="Tokyo").one()
        tokyo.city
        sess.expunge_all()

        from sqlalchemy.ext.baked import BakedQuery

        bakery = BakedQuery.bakery()

        def get_tokyo(sess):
            bq = bakery(lambda session: session.query(WeatherLocation))
            t = bq(sess).get(tokyo.id)
            return t

        Sess = sessionmaker(class_=Session, bind=db2,
                                      autoflush=True, autocommit=False)
        sess2 = Sess()

        t = get_tokyo(sess)
        eq_(t.city, tokyo.city)

        t = get_tokyo(sess2)
        eq_(t.city, tokyo.city)
```

This bug is not related to database drivers / dialects.

Questions about the fix:

1. Do we really want to cache a ShardedQuery instead of a Query in the bakery?
2. If yes, do we want to reuse a ShardedQuery for a Query? We can simply add the query class to cache key if we forbid this.

I am willing to work on the fix if I know the directions. If it is easier to fix it immediately, we can just save the trouble (of back and forth discussions about the fix). Thanks!