[Sqlalchemy-tickets] Issue #3582: Possible bug with polymorphic entities and `exists` (zzzeek/sqlalchemy)

[Yegor R. <iss...@bi...>]

New issue 3582: Possible bug with polymorphic entities and `exists`
https://bitbucket.org/zzzeek/sqlalchemy/issues/3582/possible-bug-with-polymorphic-entities-and

Yegor Roganov:

Given class hierarchy from http://docs.sqlalchemy.org/en/rel_1_0/changelog/migration_10.html
```python
class Widget(Base):
    __tablename__ = 'widget'
    id = Column(Integer, primary_key=True)
    type = Column(String)
    related_id = Column(ForeignKey('related.id'))
    related = relationship("Related", backref="widget")
    __mapper_args__ = {'polymorphic_on': type}


class FooWidget(Widget):
    __mapper_args__ = {'polymorphic_identity': 'foo'}


class Related(Base):
    __tablename__ = 'related'
    id = Column(Integer, primary_key=True)
``` 
query 
`session.query(exists().where(and_(FooWidget.related_id == 5, FooWidget.id == 6)))`
produces strange SQL (`widget.type IN (:type_1)` should be in the subquery):
```sql
SELECT EXISTS (SELECT * 
FROM widget 
WHERE widget.related_id = :related_id_1 AND widget.id = :id_1) AS anon_1 
FROM widget 
WHERE widget.type IN (:type_1)
```
Query `session.query(session.query(FooWidget).filter(and_(FooWidget.related_id == 5, FooWidget.id == 6)).exists())` is also suboptimal:
```sql
SELECT EXISTS (SELECT 1 
FROM widget 
WHERE widget.related_id = :related_id_1 AND widget.id = :id_1 AND widget.type IN (:type_1)) AS anon_1 
FROM widget 
WHERE widget.type IN (:type_1)
```
(note the repeating `widget.type IN (:type_1)` clause)


Not sure if this is a bug or I'm just using it wrong, but I figured I'd create an issue. Sorry for the hassle if it's not a bug.