fix performance problem in relation registry

relation/__init__.py

@@ -37,11 +37,16 @@ class Relation(Persistent):
 
     order = 0
     relevance = 1.0
+    fallback = None
 
     @classmethod
     def getPredicateName(cls):
         return '%s.%s' % (cls.__module__, cls.__name__)
 
+    @property
+    def ident(self):
+        return self.getPredicateName()
+
     def validate(self, registry=None):
         return True
 

relation/registry.py

@@ -186,7 +186,7 @@ class RelationRegistry(Catalog):
                 if value is not None:
                     criteria[attr] = intids.getId(value)
             pn = example.getPredicateName()
-            if pn:
+            if pn is not None:
                 criteria['relationship'] = pn
         for k in kw:
             # overwrite example fields with explicit values
@@ -244,12 +244,20 @@ def getRelations(first=None, second=None, third=None, relationships=None):
     if not relationships:
         return registry.query(**query)
     else:
-        result = set()
+        predicates = []
         for r in relationships:
-            query['relationship'] = r
-            result.update(registry.query(**query))
+            if hasattr(r, 'predicate'):
+                predicates.append(r.predicate)
+                r.predicate = None
+            else:
+                predicates.append(r.getPredicateName())
+        result = registry.query(**query)
+        if predicates:
+            return [r for r in result 
+                        if r.ident in predicates or r.fallback in predicates]
         return result
 
+
 def getRelationSingle(obj=None, relationship=None, forSecond=True):
     """ Returns the one and only relation for first having relationship
         or None if there is none.