[Sqlalchemy-tickets] Issue #4033: dont crash for add_columns() when subqueryload=True (zzzeek/sqlalchemy)

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

New issue 4033: dont crash for add_columns() when subqueryload=True
https://bitbucket.org/zzzeek/sqlalchemy/issues/4033/dont-crash-for-add_columns-when

Michael Bayer:

#4032 fixes this for the .exists() case, however at the moment a simple call to add_columns() makes subqueryload break:


```
#!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)
    bs = relationship("B", lazy="subquery")


class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_id = Column(ForeignKey('a.id'))

s = Session()

print s.query(A).add_columns('1').statement


```

for the test, make sure all of .exists() and .subquery() can succeed; do for all eager load styles.