[Sqlalchemy-tickets] Issue #3289: Aliased join with subselect does not look at load options (zzzeek/sqlalchemy)

[malthe <iss...@bi...>]

New issue 3289: Aliased join with subselect does not look at load options
https://bitbucket.org/zzzeek/sqlalchemy/issue/3289/aliased-join-with-subselect-does-not-look

malthe:

The following program outputs:
```sql
SELECT b.b_id AS b_b_id,
       b.a_id AS b_a_id,
       c.id AS c_id,
       c.c_name AS c_c_name,
       c.b_id AS c_b_id
FROM c JOIN (
       SELECT a.id AS a_id,
              a.a_name AS a_a_name,
              b.id AS b_id,
              b.b_name AS b_b_name
       FROM a JOIN b ON a.id = b.id
) AS b ON b.b_id = c.b_id
```
Note how ``b.b_name`` is in the subselect, but not used outside. It's a minor issue because it will most probably be optimized out by the database, but it still makes the query longer than it needs to be.

```python
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class A(Base):
    __tablename__ = 'a'

    id = Column(Integer, primary_key=True)
    a_name = Column(String)


class B(A):
    __tablename__ = 'b'

    id = Column(Integer, ForeignKey("a.id"), primary_key=True)
    b_name = Column(String)


class C(Base):
    __tablename__ = 'c'

    id = Column(Integer, primary_key=True)
    c_name = Column(String)
    b_id = Column("b_id", Integer, ForeignKey("b.id"))

    b = relationship(B)

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)

b = B(id=1, a_name="foo", b_name="bar")
s.add(b)

c = C(id=1, b_id=b.id, c_name="boo")
s.add(c)

s.flush()

alias = aliased(B, name="b")
print s.query(C).join(alias).options(
    contains_eager(C.b, alias=alias).load_only(alias.id)
)
```