[Sqlalchemy-tickets] Issue #3590: Eagerload is ignored if root object is already in session (zzzeek/sqlalchemy)

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

New issue 3590: Eagerload is ignored if root object is already in session
https://bitbucket.org/zzzeek/sqlalchemy/issues/3590/eagerload-is-ignored-if-root-object-is

Milas Bowman:

I have a relationship defaulted to `lazy='noload'` and on specific queries override this using `.options(eagerload())`. However, if the object being queried is already loaded into the session, it will not perform the eager load if queried.

This is a regression in 1.0.9 -- it works properly in 1.0.8. I'm specifically seeing it in a unit test using SQLite in-memory database.

Here's a simple example (also attached):
```
#!python
from __future__ import print_function

from sqlalchemy import Column, Integer, ForeignKey, String, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import eagerload, relationship, sessionmaker

Base = declarative_base()
engine = create_engine('sqlite:///:memory:', echo=True)
Session = sessionmaker(bind=engine)


class Foo(Base):
    __tablename__ = 'foo'

    id = Column(Integer, primary_key=True)
    value = Column(String(50))
    bars = relationship('Bar', back_populates='foo', lazy='noload')


class Bar(Base):
    __tablename__ = 'bar'

    id = Column(Integer, primary_key=True)
    foo_id = Column(ForeignKey('foo.id'))
    foo = relationship('Foo', back_populates='bars', lazy='noload')


def initialize():
    Base.metadata.create_all(engine)

    session = Session()
    f = Foo(value='foobar')
    b = Bar(foo=f)
    session.add(b)
    session.commit()
    session.close()

if __name__ == '__main__':
    initialize()

    session = Session()

    bar = session.query(Bar).one()
    print('Foo should be None - ', bar.foo)

    bar_with_foo_query = session.query(Bar).options(eagerload(Bar.foo))

    bar_missing_foo = bar_with_foo_query.one()
    print('Foo is incorrectly None - ', bar_missing_foo.foo)

    # invalidate the session cache to force it to reload
    session.close()

    bar_with_foo = bar_with_foo_query.one()
    print('Foo is loaded correctly - ', bar_with_foo.foo)
```

Let me know if there's any additional information I can provide.