[Sqlalchemy-tickets] Issue #3583: `obj.foo = set(obj.foo)` (zzzeek/sqlalchemy)

[thiefmaster <iss...@bi...>]

New issue 3583: `obj.foo = set(obj.foo)`
https://bitbucket.org/zzzeek/sqlalchemy/issues/3583/objfoo-set-objfoo

thiefmaster:

I have an `association_proxy` on a set-like relationship. Normal set operations work perfectly when it comes to not adding duplicates, but when I replace the whole set e.g. using `foo.bar = set(foo.bar)` (in my real application the right side comes from a form) I get errors due to duplicate rows being inserted.

```python
from sqlalchemy import *
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import *


Base = declarative_base()


class A(Base):
    __tablename__ = 'test_a'

    id = Column(Integer, primary_key=True)
    b_rel = relationship('B', collection_class=set, cascade='all, delete-orphan')
    b = association_proxy('b_rel', 'value', creator=lambda x: B(value=x))


class B(Base):
    __tablename__ = 'test_b'
    __table_args__ = UniqueConstraint('a_id', 'value'),

    id = Column(Integer, primary_key=True)
    a_id = Column(Integer, ForeignKey('test_a.id'), nullable=False)
    value = Column(String)


e = create_engine('sqlite:///:memory:', echo=True)
# e = create_engine('postgresql:///test', echo=True)
Base.metadata.create_all(e)
# e.execute('TRUNCATE test_a, test_b;')
s = Session(e)

a = A()
a.b = {'x', 'y', 'z'}
s.add(a)
s.commit()

print
print 'adding existing element to set'
a.b.add('x')
s.flush()

print
print 'assigning same items to set'
a.b = set(a.b)
s.flush()
```