- Improved the error message emitted when a "backref loop" is detected,

that is when an attribute event triggers a bidirectional
assignment between two other attributes with no end.
This condition can occur not just when an object of the wrong
type is assigned, but also when an attribute is mis-configured
to backref into an existing backref pair.
- A warning is emitted when a MapperProperty is assigned to a mapper
that replaces an existing property, if the properties in question
aren't plain column-based properties.   Replacement of relationship
properties is rarely (ever?) what is intended and usually refers to a
mapper mis-configuration.   This will also warn if a backref configures
itself on top of an existing one in an inheritance relationship
(which is an error in 0.8).
backport for [ticket:2674]

6 files changed, 143 insertions, 12 deletions

diff --git a/doc/build/changelog/changelog_07.rst b/doc/build/changelog/changelog_07.rst
index f07c9eccd..aeb7251db 100644
--- a/doc/build/changelog/changelog_07.rst
+++ b/doc/build/changelog/changelog_07.rst
@@ -14,6 +14,29 @@
       Fixed an import of "logging" in test_execute which was not
       working on some linux platforms.
 
+    .. change::
+        :tags: bug, orm
+        :tickets: 2674
+
+      Improved the error message emitted when a "backref loop" is detected,
+      that is when an attribute event triggers a bidirectional
+      assignment between two other attributes with no end.
+      This condition can occur not just when an object of the wrong
+      type is assigned, but also when an attribute is mis-configured
+      to backref into an existing backref pair.
+
+    .. change::
+      :tags: bug, orm
+      :tickets: 2674
+
+      A warning is emitted when a MapperProperty is assigned to a mapper
+      that replaces an existing property, if the properties in question
+      aren't plain column-based properties.   Replacement of relationship
+      properties is rarely (ever?) what is intended and usually refers to a
+      mapper mis-configuration.   This will also warn if a backref configures
+      itself on top of an existing one in an inheritance relationship
+      (which is an error in 0.8).
+
 .. changelog::
     :version: 0.7.10
     :released: Thu Feb 7 2013
diff --git a/lib/sqlalchemy/orm/attributes.py b/lib/sqlalchemy/orm/attributes.py
index b40abd57b..59cc461ff 100644
--- a/lib/sqlalchemy/orm/attributes.py
+++ b/lib/sqlalchemy/orm/attributes.py
@@ -319,6 +319,9 @@ class AttributeImpl(object):
 
         self.expire_missing = expire_missing
 
+    def __str__(self):
+        return "%s.%s" % (self.class_.__name__, self.key)
+
     def _get_active_history(self):
         """Backwards compat for impl.active_history"""
 
@@ -975,11 +978,18 @@ def backref_listeners(attribute, key, uselist):
 
     parent_token = attribute.impl.parent_token
 
-    def _acceptable_key_err(child_state, initiator):
+    def _acceptable_key_err(child_state, initiator, child_impl):
         raise ValueError(
-            "Object %s not associated with attribute of "
-            "type %s" % (mapperutil.state_str(child_state),
-                    manager_of_class(initiator.class_)[initiator.key]))
+            "Bidirectional attribute conflict detected: "
+            'Passing object %s to attribute "%s" '
+            'triggers a modify event on attribute "%s" '
+            'via the backref "%s".' % (
+                mapperutil.state_str(child_state),
+                initiator.parent_token,
+                child_impl.parent_token,
+                attribute.impl.parent_token
+            )
+        )
 
     def emit_backref_from_scalar_set_event(state, child, oldchild, initiator):
         if oldchild is child:
@@ -1002,7 +1012,7 @@ def backref_listeners(attribute, key, uselist):
             child_impl = child_state.manager[key].impl
             if initiator.parent_token is not parent_token and \
                 initiator.parent_token is not child_impl.parent_token:
-                _acceptable_key_err(state, initiator)
+                _acceptable_key_err(state, initiator, child_impl)
             child_impl.append(
                                 child_state,
                                 child_dict,
@@ -1015,9 +1025,11 @@ def backref_listeners(attribute, key, uselist):
         child_state, child_dict = instance_state(child), \
                                     instance_dict(child)
         child_impl = child_state.manager[key].impl
+
+        print initiator.parent_token, parent_token, child_impl.parent_token
         if initiator.parent_token is not parent_token and \
             initiator.parent_token is not child_impl.parent_token:
-            _acceptable_key_err(state, initiator)
+            _acceptable_key_err(state, initiator, child_impl)
         child_impl.append(
                                 child_state,
                                 child_dict,
diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index de4d351b0..36771de23 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -1101,7 +1101,7 @@ class Mapper(object):
                 # initialized; check for 'readonly'
                 if hasattr(self, '_readonly_props') and \
                     (not hasattr(col, 'table') or
-                    col.table not in self._cols_by_table):
+                        col.table not in self._cols_by_table):
                         self._readonly_props.add(prop)
 
             else:
@@ -1139,6 +1139,16 @@ class Mapper(object):
                         "%r for column %r" % (syn, key, key, syn)
                     )
 
+        if key in self._props and \
+                not isinstance(prop, properties.ColumnProperty) and \
+                not isinstance(self._props[key], properties.ColumnProperty):
+            util.warn("Property %s on %s being replaced with new "
+                            "property %s; the old property will be discarded" % (
+                            self._props[key],
+                            self,
+                            prop,
+                        ))
+
         self._props[key] = prop
 
         if not self.non_primary:
diff --git a/test/orm/test_attributes.py b/test/orm/test_attributes.py
index 7a4ef038e..fecd23c93 100644
--- a/test/orm/test_attributes.py
+++ b/test/orm/test_attributes.py
@@ -1217,7 +1217,10 @@ class CyclicBackrefAssertionTest(fixtures.TestBase):
         b1 = B()
         assert_raises_message(
             ValueError,
-            "Object <B at .*> not associated with attribute of type C.a",
+            'Bidirectional attribute conflict detected: '
+            'Passing object <B at .*> to attribute "C.a" '
+            'triggers a modify event on attribute "C.b" '
+            'via the backref "B.c".',
             setattr, c1, 'a', b1
         )
 
@@ -1227,10 +1230,14 @@ class CyclicBackrefAssertionTest(fixtures.TestBase):
         b1 = B()
         assert_raises_message(
             ValueError,
-            "Object <B at .*> not associated with attribute of type C.a",
+            'Bidirectional attribute conflict detected: '
+            'Passing object <B at .*> to attribute "C.a" '
+            'triggers a modify event on attribute "C.b" '
+            'via the backref "B.c".',
             c1.a.append, b1
         )
 
+
     def _scalar_fixture(self):
         class A(object):
             pass
@@ -1272,6 +1279,36 @@ class CyclicBackrefAssertionTest(fixtures.TestBase):
 
         return A, B, C
 
+    def _broken_collection_fixture(self):
+        class A(object):
+            pass
+        class B(object):
+            pass
+        instrumentation.register_class(A)
+        instrumentation.register_class(B)
+
+        attributes.register_attribute(A, 'b', backref='a1', useobject=True)
+        attributes.register_attribute(B, 'a1', backref='b', useobject=True,
+                                                uselist=True)
+
+        attributes.register_attribute(B, 'a2', backref='b', useobject=True,
+                                                uselist=True)
+
+        return A, B
+
+    def test_broken_collection_assertion(self):
+        A, B = self._broken_collection_fixture()
+        b1 = B()
+        a1 = A()
+        assert_raises_message(
+            ValueError,
+            'Bidirectional attribute conflict detected: '
+            'Passing object <A at .*> to attribute "B.a2" '
+            'triggers a modify event on attribute "B.a1" '
+            'via the backref "A.b".',
+            b1.a2.append, a1
+        )
+
 class PendingBackrefTest(fixtures.ORMTest):
     def setup(self):
         global Post, Blog, called, lazy_load
diff --git a/test/orm/test_compile.py b/test/orm/test_compile.py
index 1b2714d70..9e058e372 100644
--- a/test/orm/test_compile.py
+++ b/test/orm/test_compile.py
@@ -167,8 +167,10 @@ class CompileTest(fixtures.ORMTest):
         b = Table('b', meta, Column('id', Integer, primary_key=True),
                                 Column('a_id', Integer, ForeignKey('a.id')))
 
-        class A(object):pass
-        class B(object):pass
+        class A(object):
+            pass
+        class B(object):
+            pass
 
         mapper(A, a, properties={
             'b':relationship(B, backref='a')
@@ -183,3 +185,32 @@ class CompileTest(fixtures.ORMTest):
             configure_mappers
         )
 
+    def test_conflicting_backref_subclass(self):
+        meta = MetaData()
+
+        a = Table('a', meta, Column('id', Integer, primary_key=True))
+        b = Table('b', meta, Column('id', Integer, primary_key=True),
+                                Column('a_id', Integer, ForeignKey('a.id')))
+
+        class A(object):
+            pass
+        class B(object):
+            pass
+        class C(B):
+            pass
+
+        mapper(A, a, properties={
+            'b': relationship(B, backref='a'),
+            'c': relationship(C, backref='a')
+        })
+        mapper(B, b)
+        mapper(C, None, inherits=B)
+
+        # see [ticket:2674] - 0.8 has an expanded check for
+        # conflicting backrefs that raises an ArgumentError
+        assert_raises_message(
+            sa_exc.SAWarning,
+            "Property C.a on Mapper|C|b being replaced with new "
+            "property B.a; the old property will be discarded",
+            configure_mappers
+        )
diff --git a/test/orm/test_mapper.py b/test/orm/test_mapper.py
index 5a88d937f..39e653357 100644
--- a/test/orm/test_mapper.py
+++ b/test/orm/test_mapper.py
@@ -447,7 +447,7 @@ class MapperTest(_fixtures.FixtureTest, AssertsCompiledSQL):
         assert hasattr(User, 'addresses')
         assert "addresses" in [p.key for p in m1._polymorphic_properties]
 
-    def test_replace_property(self):
+    def test_replace_col_prop_w_syn(self):
         users, User = self.tables.users, self.classes.User
 
         m = mapper(User, users)
@@ -473,6 +473,24 @@ class MapperTest(_fixtures.FixtureTest, AssertsCompiledSQL):
         u.name = 'jacko'
         assert m._columntoproperty[users.c.name] is m.get_property('_name')
 
+    def test_replace_rel_prop_with_rel_warns(self):
+        users, User = self.tables.users, self.classes.User
+        addresses, Address = self.tables.addresses, self.classes.Address
+
+        m = mapper(User, users, properties={
+                "addresses": relationship(Address)
+            })
+        mapper(Address, addresses)
+
+        assert_raises_message(
+            sa.exc.SAWarning,
+            "Property User.addresses on Mapper|User|users being replaced "
+            "with new property User.addresses; the old property will "
+            "be discarded",
+            m.add_property,
+            "addresses", relationship(Address)
+        )
+
     def test_add_column_prop_deannotate(self):
         User, users = self.classes.User, self.tables.users
         Address, addresses = self.classes.Address, self.tables.addresses