[Sqlalchemy-tickets] Issue #4199: selectin polymorphic hitting expanding in w/ no entries (zzzeek/sqlalchemy)

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

New issue 4199: selectin polymorphic hitting expanding in w/ no entries
https://bitbucket.org/zzzeek/sqlalchemy/issues/4199/selectin-polymorphic-hitting-expanding-in

Michael Bayer:

```
#!python

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import event

Base = declarative_base()


class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)
    type = Column(String)
    b_id = Column(ForeignKey('b.id'))

    __mapper_args__ = {
        'polymorphic_on': type,
    }


class A1(A):
    __tablename__ = 'a1'
    id = Column(Integer, ForeignKey('a.id'), primary_key=True)
    __mapper_args__ = {
        'polymorphic_identity': 'a1',
        'polymorphic_load': 'selectin',
    }


class A2(A):
    __tablename__ = 'a2'
    id = Column(Integer, ForeignKey('a.id'), primary_key=True)
    __mapper_args__ = {
        'polymorphic_identity': 'a2',
        'polymorphic_load': 'selectin',
    }


class B(Base):
    __tablename__ = 'b'
    id = Column(Integer, primary_key=True)
    a_list = relationship('A')


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

s = Session(e)

b = B(a_list=[A1(), A2()])
s.add(b)
x = b.a_list[0]

# uncomment for pass
#y = b.a_list[1]

s.commit()  # L3

print(b.a_list)  # crashes
```


```
#!python

File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/engine/default.py", line 736, in _expand_in_parameters
    "'expanding' parameters can't be used with an "
sqlalchemy.exc.StatementError: (sqlalchemy.exc.InvalidRequestError) 'expanding' parameters can't be used with an empty list [SQL: 'SELECT a1.id AS a1_id, a.id AS a_id, a.type AS a_type \nFROM a JOIN a1 ON a.id = a1.id \nWHERE a.id IN ([EXPANDING_primary_keys]) ORDER BY a.id'] [parameters: [{'primary_keys': []}]]

```