retrieval: fix cache cleaning

fixes sqlalchemy detachted session problem

retrieval/retrieval.py

@@ -51,7 +51,8 @@ class Retrieval():
         """See retrieval.create"""
         entity = self._structure[table]
         assert(len(args) == len(entity.parameter))
-        # konstruct keys
+        # map argument values to entity.query_filter keywords
+        # needed for unique identification
         kwargs = entity.query_filter.get_kwargs()
         for key in kwargs:
             kwargs[key] = args[entity.parameter.index(kwargs[key])]
@@ -85,8 +86,8 @@ class Retrieval():
     def commit(self):
         """See retrieval.commit"""
         with session_scope(self._Session) as session:
-            for table in self._cache.values():
-                for entity in table.values():
+            for table in self._cache:
+                for entity in self._cache[table].values():
                     session.add(entity)
                 # clean the cache
-                table = {}
+                self._cache[table] = {}