[Sqlalchemy-tickets] Issue #4175: selectinload raises an exception when used with sharded sessions (zzzeek/sqlalchemy)

[Rishi S. <iss...@bi...>]

New issue 4175: selectinload raises an exception when used with sharded sessions
https://bitbucket.org/zzzeek/sqlalchemy/issues/4175/selectinload-raises-an-exception-when-used

Rishi Sharma:

```
#!python

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.horizontal_shard import ShardedSession

Base = declarative_base()


class Book(Base):
    __tablename__ = 'book'
    id = Column(Integer, primary_key=True)
    pages = relationship('Page')


class Page(Base):
    __tablename__ = 'page'
    id = Column(Integer, primary_key=True)
    book_id = Column(ForeignKey('book.id'))


def test_query_with_selectinload(session):
    book = Book()
    book.pages.append(Page())

    session.add(book)
    session.commit()

    session.query(Book).options(selectinload('pages')).all()

if __name__ == "__main__":
    engine = create_engine("mysql://root:@127.0.0.1:3306/test", echo=True)
    Base.metadata.create_all(engine)

    # vanilla_session = Session(engine)
    # test_query_with_selectinload(vanilla_session)

    sharded_session = scoped_session(
        sessionmaker(
            class_=ShardedSession,
        )
    )

    sharded_session.configure(
        shards=dict(test=engine),
        shard_chooser=lambda *args: 'test',
        id_chooser=lambda *args: None,
        query_chooser=lambda *args: ['test']
    )

    test_query_with_selectinload(sharded_session)

```
produces the following stack trace:


```
#!text

Traceback (most recent call last):
  File "selectinload_horizontal_shard_query_test_case.py", line 50, in <module>
    test_query_with_selectinload(sharded_session)
  File "selectinload_horizontal_shard_query_test_case.py", line 28, in test_query_with_selectinload
    session.query(Book).options(selectinload('pages')).all()
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2726, in all
    return list(self)
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2878, in __iter__
    return self._execute_and_instances(context)
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/ext/horizontal_shard.py", line 59, in _execute_and_instances
    partial.extend(iter_for_shard(shard_id))
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/orm/loading.py", line 97, in instances
    util.raise_from_cause(err)
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/orm/loading.py", line 85, in instances
    post_load.invoke(context, path)
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/orm/loading.py", line 759, in invoke
    context, path, states, self.load_keys, *arg, **kw)
  File "<string>", line 1, in <lambda>
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/orm/strategies.py", line 1966, in _load_for_path
    lambda x: x[0]
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/ext/baked.py", line 341, in __iter__
    return q._execute_and_instances(context)
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/ext/horizontal_shard.py", line 59, in _execute_and_instances
    partial.extend(iter_for_shard(shard_id))
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/ext/horizontal_shard.py", line 48, in iter_for_shard
    mapper=self._mapper_zero(),
  File "/Users/rsharma/src/evolve/env/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 328, in _mapper_zero
    return self._entities[0].mapper
AttributeError: '_BundleEntity' object has no attribute 'mapper'
```