Add use_lists arg for cache.cache_model(), plus CacheKeyNotSupported

rattail/db/cache.py

@@ -1,8 +1,8 @@
-# -*- coding: utf-8 -*-
+# -*- coding: utf-8; -*-
 ################################################################################
 #
 #  Rattail -- Retail Software Framework
-#  Copyright © 2010-2015 Lance Edgar
+#  Copyright © 2010-2017 Lance Edgar
 #
 #  This file is part of Rattail.
 #
@@ -37,13 +37,22 @@ from rattail.db import model
 log = logging.getLogger(__name__)
 
 
+class CacheKeyNotSupported(Exception):
+    """
+    Special error which a model cacher should raise when generating the cache
+    key for a given record/object, but when it is discovered the object does
+    not have sufficient data to generate the key.
+    """
+
+
 class ModelCacher(object):
     """
     Generic model data caching class.
     """
 
     def __init__(self, session, model_class, key='uuid', omit_duplicates=False,
-                 query=None, order_by=None, query_options=None, normalizer=None):
+                 query=None, order_by=None, query_options=None, normalizer=None,
+                 use_lists=False):
         self.session = session
         self.model_class = model_class
         self.key = key
@@ -56,6 +65,7 @@ class ModelCacher(object):
             self.normalize = lambda d: d
         else:
             self.normalize = normalizer
+        self.use_lists = use_lists
     
     @property
     def model_name(self):
@@ -107,7 +117,14 @@ class ModelCacher(object):
 
     def cache_instance(self, instance):
         normalized = self.normalize(instance)
+        try:
             key = self.get_key(instance, normalized)
+        except CacheKeyNotSupported:
+            # this means the object doesn't belong in our cache
+            return
+        if self.use_lists:
+            self.instances.setdefault(key, []).append(normalized)
+        else:
             if key not in self.instances:
                 self.instances[key] = normalized
             else: