Added support to search for null fields in Solr

haystack/backends/solr_backend.py

@@ -422,12 +422,19 @@ def build_query_fragment(self, field, filter_type, value):
         if hasattr(value, 'values_list'):
             value = list(value)
 
+        index_fieldname = connections[self._using].get_unified_index().get_index_fieldname(field)
+
+        if value is None:
+            # The filter is for a document field with the value of None.
+            # As an optimization, we wouldn't have stored that field on the document,
+            # so we're really looking for documents without this field at all.
+            # Return solr's search filter for the empty fields
+            return ': -%s:[* TO *]' % index_fieldname
+
         if not isinstance(value, (set, list, tuple)):
             # Convert whatever we find to what pysolr wants.
             value = self.backend.conn._from_python(value)
 
-        index_fieldname = connections[self._using].get_unified_index().get_index_fieldname(field)
-
         filter_types = {
             'contains': u'%s:%s',
             'exact': u'%s:"%s"',