Deleting members of a relation() collection via del causes constraint violations.

Consider the following:

from sqlalchemy import create_engine,Column,Integer,ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker,relation
from sqlalchemy.orm.interfaces import AttributeExtension


c=create_engine("postgresql://bryant:uugoh40u@localhost/antekdb",echo=True)
Base=declarative_base()
Base.metadata.bind=c
Session=sessionmaker(bind=c)

class B(Base):
    __tablename__="b"

    id=Column(Integer,primary_key=True)
    aid=Column(Integer,ForeignKey("a.id"),nullable=False)

class A(Base):
    __tablename__="a"

    id=Column(Integer,primary_key=True)
    b=relation("B",backref="a")

Base.metadata.drop_all()
Base.metadata.create_all()
s=Session()

a=A()
a.b=[B(),B(),B()](B(),B(),B())
s.add(a)
s.commit()

a=s.query(A).first()

## impending weirdness

## what I want
for i in a.b:
    s.delete(i);

## integrity error
del a.b[:](:)


s.commit()

Is it an intended feature for "del a.b..." to generate UPDATE statements with NULL values in the ForeignKey? If one is not supposed to del a.b:, then perhaps an error should be generated on the sqlalchemy level, instead of relying on the database to squawk?

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