honor passive on scalar backref assignment

patch:

Index: lib/sqlalchemy/orm/dynamic.py
===================================================================
--- lib/sqlalchemy/orm/dynamic.py   (revision 6172)
+++ lib/sqlalchemy/orm/dynamic.py   (working copy)
@@ -100,7 +100,7 @@
         dict_[self.key](self.key) = True
         return state.committed_state[self.key](self.key)

-    def set(self, state, dict_, value, initiator):
+    def set(self, state, dict_, value, initiator, passive=attributes.PASSIVE_OFF):
         if initiator is self:
             return

Index: lib/sqlalchemy/orm/attributes.py
===================================================================
--- lib/sqlalchemy/orm/attributes.py    (revision 6172)
+++ lib/sqlalchemy/orm/attributes.py    (working copy)
@@ -383,12 +383,12 @@
             return self.initialize(state, dict_)

     def append(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
-        self.set(state, dict_, value, initiator)
+        self.set(state, dict_, value, initiator, passive=passive)

     def remove(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
-        self.set(state, dict_, None, initiator)
+        self.set(state, dict_, None, initiator, passive=passive)

-    def set(self, state, dict_, value, initiator):
+    def set(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
         raise NotImplementedError()

     def get_committed_value(self, state, dict_, passive=PASSIVE_OFF):
@@ -436,7 +436,7 @@
         return History.from_attribute(
             self, state, dict_.get(self.key, NO_VALUE))

-    def set(self, state, dict_, value, initiator):
+    def set(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
         if initiator is self:
             return

@@ -511,7 +511,7 @@
         ScalarAttributeImpl.delete(self, state, dict_)
         state.mutable_dict.pop(self.key)

-    def set(self, state, dict_, value, initiator):
+    def set(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
         if initiator is self:
             return

@@ -559,7 +559,7 @@
             else:
                 return History.from_attribute(self, state, current)

-    def set(self, state, dict_, value, initiator):
+    def set(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
         """Set a value on the given InstanceState.

         `initiator` is the ``InstrumentedAttribute`` that initiated the
@@ -570,8 +570,10 @@
         if initiator is self:
             return

-        # may want to add options to allow the get() here to be passive
-        old = self.get(state, dict_)
+        old = self.get(state, dict_, passive=passive)
+        if old is PASSIVE_NORESULT:
+            old = None
+             
         value = self.fire_replace_event(state, dict_, value, old, initiator)
         dict_[self.key](self.key) = value

@@ -707,7 +709,7 @@
         else:
             collection.remove_with_event(value, initiator)

-    def set(self, state, dict_, value, initiator):
+    def set(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
         """Set a value on the given object.

         `initiator` is the ``InstrumentedAttribute`` that initiated the

here is a test for the common case, throws the usual orphan error without the patch:

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


Base = declarative_base()


class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    name = Column(String)

    site = relation("Site", uselist=False, cascade="all, delete-orphan", backref="user")

class Site(Base):
    __tablename__ = 'site'
    id = Column(Integer, primary_key=True)
    user_id = Column(Integer, ForeignKey('user.id'))
    title = Column(String)

engine = create_engine('sqlite://', echo=True)

Base.metadata.create_all(engine)

sess = sessionmaker(bind=engine)()
v = User(name='victor')
sess.add(v)
sess.commit()

print "---------------"
s = Site(title='asdf')
sess.add(s)
s.user = v
sess.commit()

however, need to ensure things go well for every combination of forwards/backwards ref here. Also is None the most appropriate thing to be using here (i.e. not NO_VALUE, etc.)

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