can we get subclass mappers to self-collect?

would need a lot of weak references for this to work automatically, particularly the memoizations somehow, which would add some real performance overhead:
diff -r 45007b9c2effe931ca4493b25171fb9a9c244611 lib/sqlalchemy/ext/declarative.py
--- a/lib/sqlalchemy/ext/declarative.py Mon Jun 25 16:42:39 2012 -0400
+++ b/lib/sqlalchemy/ext/declarative.py Tue Jun 26 12:58:56 2012 -0400
@@ -1036,7 +1036,7 @@
 from sqlalchemy.sql import util as sql_util, expression
 from sqlalchemy import event
 from sqlalchemy.orm.util import polymorphic_union, _mapper_or_none
-
+import weakref

 __all__ = 'declarative_base', 'synonym_for', \
             'comparable_using', 'instrument_declarative'
@@ -1718,7 +1718,7 @@
         lcl_metadata.bind = bind

     if class_registry is None:
-        class_registry = {}
+        class_registry = weakref.WeakValueDictionary()

     bases = not isinstance(cls, tuple) and (cls,) or cls
     class_dict = dict(_decl_class_registry=class_registry,
diff -r 45007b9c2effe931ca4493b25171fb9a9c244611 lib/sqlalchemy/orm/mapper.py
--- a/lib/sqlalchemy/orm/mapper.py  Mon Jun 25 16:42:39 2012 -0400
+++ b/lib/sqlalchemy/orm/mapper.py  Tue Jun 26 12:58:56 2012 -0400
@@ -166,7 +166,7 @@
         # names with Mappers that will be used to construct object instances
         # upon a select operation.
         if _polymorphic_map is None:
-            self.polymorphic_map = {}
+            self.polymorphic_map = weakref.WeakValueDictionary()
         else:
             self.polymorphic_map = _polymorphic_map

@@ -432,7 +432,8 @@
         being present."""

         # a set of all mappers which inherit from this one.
-        self._inheriting_mappers = set()
+        # TODO: WeakSet is only in 2.7, would need compat
+        self._inheriting_mappers = weakref.WeakSet()

         if self.inherits:
             if isinstance(self.inherits, type):



#!python
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base= declarative_base()

class Root(Base):
    __tablename__ = 'root'
    id = Column(Integer, primary_key=True)
    type = Column(String)
    __mapper_args__ = {
        'polymorphic_on':type,
        'polymorphic_identity':'root'
    }

assert not Root.__mapper__._inheriting_mappers

class Sub(Root):
    __tablename__ = 'sub'
    metadata = MetaData()
    id = Column(Integer, ForeignKey(Root.id), primary_key=True)
    __mapper_args__ = {
        'polymorphic_identity':'sub'
    }

e = create_engine("sqlite://")
Base.metadata.create_all(e)
Sub.metadata.create_all(e)
compile_mappers()
s = Session(e)

s.add_all([Sub()](Root(),))
print s.query(Root).all()

del Sub

# this step is required, though, unless we 
# make the memoizations weak too, that would
# be a real performance hit
Root.__mapper__._expire_memoizations()

import gc
gc.collect()
assert not Root.__mapper__._inheriting_mappers
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