Patch pymongo a bit, so that the hash of the DBRef is not computed all

the time but cached. DBRef's are generally considered immutable.

src/mongopersist/__init__.py

@@ -1 +1,5 @@
 # Make a package.
+
+from . import pymongo
+pymongo.patch()
+del pymongo

src/mongopersist/pymongo.py

@@ -0,0 +1,32 @@
+##############################################################################
+#
+# Copyright (c) 2011 Zope Foundation and Contributors.
+# All Rights Reserved.
+#
+# This software is subject to the provisions of the Zope Public License,
+# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
+# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
+# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
+# FOR A PARTICULAR PURPOSE.
+#
+##############################################################################
+"""PyMongo Patches"""
+
+def lazy_hash(self):
+    """Get a hash value for this :class:`DBRef`.
+
+    .. versionadded:: 1.1
+    """
+    if self.__hash is None:
+        self.__hash = self.orig_hash()
+    return self.__hash
+
+def patch():
+    # ObjectId should get patched too, but it is hard, since it uses slots
+    # *and* rquires the original object reference to be around (otherwise it
+    # creates BSON encoding errors.
+    import bson.dbref
+    bson.dbref.DBRef.__hash = None
+    bson.dbref.DBRef.orig_hash = bson.dbref.DBRef.__hash__
+    bson.dbref.DBRef.__hash__ = lazy_hash