Fixed #13640: Avoid generating an exception when a model has an attribute named 'evaluate'. Thanks LukaszKorzybski and tobias.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17093 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/sql/query.py

@@ -14,6 +14,7 @@ from django.utils.encoding import force_unicode
 from django.utils.tree import Node
 from django.db import connections, DEFAULT_DB_ALIAS
 from django.db.models import signals
+from django.db.models.expressions import ExpressionNode
 from django.db.models.fields import FieldDoesNotExist
 from django.db.models.query_utils import InvalidQuery
 from django.db.models.sql import aggregates as base_aggregates_module
@@ -1064,7 +1065,7 @@ class Query(object):
             value = True
         elif callable(value):
             value = value()
-        elif hasattr(value, 'evaluate'):
+        elif isinstance(value, ExpressionNode):
             # If value is a query expression, evaluate it
             value = SQLEvaluator(value, self)
             having_clause = value.contains_aggregate