Re: [Sqlalchemy-tickets] [sqlalchemy] #2741: uselist=False collection not backref deferred, leads to unwanted load/autoflush

[sqlalchemy <mi...@zz...>]

#2741: uselist=False collection not backref deferred, leads to unwanted
load/autoflush
---------------------------+-------------------------------
      Reporter:  zzzeek    |      Owner:  zzzeek
          Type:  defect    |     Status:  new
      Priority:  highest   |  Milestone:  0.8.xx
     Component:  orm       |   Severity:  major - 1-3 hours
    Resolution:            |   Keywords:
Progress State:  in queue  |
---------------------------+-------------------------------

Comment (by zzzeek):

 the difference between uselist=True and uselist=False is that the old
 value must be *removed*.  Plenty of tests in test_backref_mutations test
 this kind of thing.  Suppose we disable the load on backref:

 {{{
 #!diff
 diff --git a/lib/sqlalchemy/orm/attributes.py
 b/lib/sqlalchemy/orm/attributes.py
 index bfba695..4ff6261 100644
 --- a/lib/sqlalchemy/orm/attributes.py
 +++ b/lib/sqlalchemy/orm/attributes.py
 @@ -784,7 +784,7 @@ class ScalarObjectAttributeImpl(ScalarAttributeImpl):
          if initiator and initiator.parent_token is self.parent_token:
              return

 -        if self.dispatch._active_history:
 +        if passive is PASSIVE_OFF and self.dispatch._active_history:
              old = self.get(state, dict_, passive=PASSIVE_ONLY_PERSISTENT)
          else:
              old = self.get(state, dict_, passive=PASSIVE_NO_FETCH)
 }}}

 then run the test like this:

 {{{
 #!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)
     b = relationship("B", uselist=False, backref="a")

 class B(Base):
     __tablename__ = 'b'

     id = Column(Integer, primary_key=True)
     a_id = Column(Integer, ForeignKey('a.id'), nullable=False)

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

 sess = Session(e)

 a1 = A()
 b1 = B(a=a1)
 a2 = A()
 b2 = B(a=a2)

 sess.add_all([a1, b1, a2, b2])
 sess.commit()

 b1.a = a2

 sess.commit()

 print a2.b
 }}}

 `a2.b` now has *two* b's attached and `b2` has not been replaced by `b1`,
 we get:

 {{{
 SELECT b.id AS b_id, b.a_id AS b_a_id
 FROM b
 WHERE ? = b.a_id
 2013-06-04 12:28:23,543 INFO sqlalchemy.engine.base.Engine (2,)
 2013-06-04 12:28:23,543 DEBUG sqlalchemy.engine.base.Engine Col ('b_id',
 'b_a_id')
 2013-06-04 12:28:23,543 DEBUG sqlalchemy.engine.base.Engine Row (1, 2)
 2013-06-04 12:28:23,543 DEBUG sqlalchemy.engine.base.Engine Row (2, 2)
 /Users/classic/dev/sqlalchemy/lib/sqlalchemy/orm/strategies.py:524:
 SAWarning: Multiple rows returned with uselist=False for lazily-loaded
 attribute 'A.b'
   return self._emit_lazyload(session, state, ident_key, passive)
 <__main__.B object at 0x10161f7d0>
 }}}

 in this case, if we stick to the plan and do no_autoflush:

 {{{
 with sess.no_autoflush:
     b1.a = a2

 sess.commit()

 print a2.b
 }}}

 we still get an integrity error, because the replacement of `b2` throws a
 not null exception, but this is what we want in this case, since
 uselist=False means the old value must be replaced.

--
Ticket URL: <http://www.sqlalchemy.org/trac/ticket/2741#comment:1>
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