[Sqlalchemy-tickets] Issue #4373: Load.load_only on polymorphic child doesn't work (zzzeek/sqlalchemy)

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

New issue 4373: Load.load_only on polymorphic child doesn't work
https://bitbucket.org/zzzeek/sqlalchemy/issues/4373/loadload_only-on-polymorphic-child-doesnt

Dmytro Starosud:

This is using SQLAlchemy==1.2.14.
Please consider following code snippet:

```
#!python
from sqlalchemy import Column, Integer, String, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import Load, Query
from sqlalchemy.pool import NullPool

engine = create_engine('postgresql://***', poolclass=NullPool)
connection = engine.connect()
connection.begin_nested()

Base = declarative_base()


class MyEntity(Base):
    __tablename__ = 'my_entity'

    id = Column(Integer, primary_key=True)

    a = Column(String, nullable=False)
    b = Column(String, nullable=False)
    kind = Column(String, nullable=False)

    __mapper_args__ = {'polymorphic_on': kind}


Base.metadata.create_all(connection)


class MyChild(MyEntity):
    __mapper_args__ = {'polymorphic_identity': 'child_1'}


print(MyEntity)
print(Query(MyEntity).options(Load(MyEntity).load_only('a')))
print(MyChild)
print(Query(MyChild).options(Load(MyChild).load_only('a')))
```

It produces following output:


```
<class '__main__.MyEntity'>
SELECT my_entity.id AS my_entity_id, my_entity.a AS my_entity_a, my_entity.kind AS my_entity_kind
FROM my_entity
<class '__main__.MyChild'>
SELECT my_entity.id AS my_entity_id, my_entity.a AS my_entity_a, my_entity.b AS my_entity_b, my_entity.kind AS my_entity_kind
FROM my_entity
WHERE my_entity.kind IN (:kind_1)
```

Please assist. Did I misread documentation? Note unbound `load_only('a')` works well.