Added better error checking for when trying to set an attribute on an object stored in an attribute but which after retrieved and cached was deleted. The fix doesn't fully fix the problem of stale caching but does give more information. Related to Issue 295.

Also fixed a curiously phrased import.

src/typeclasses/models.py

@@ -34,7 +34,7 @@ except ImportError:
 import traceback
 from collections import defaultdict
 
-from django.db import models
+from django.db import models, IntegrityError
 from django.conf import settings
 from django.utils.encoding import smart_str
 from django.contrib.contenttypes.models import ContentType
@@ -377,6 +377,7 @@ class Attribute(SharedMemoryModel):
         self.no_cache = False
         self.db_value = to_unicode(_PDUMPS(to_str(new_value)))
         self.save()
+
     #@value.deleter
     def __value_del(self):
         "Deleter. Allows for del attr.value. This removes the entire attribute."
@@ -1291,7 +1292,15 @@ class TypedObject(SharedMemoryModel):
                 # no match; create new attribute
                 attrib_obj = attrclass(db_key=attribute_name, db_obj=self)
         # re-set an old attribute value
-        attrib_obj.value = new_value
+        try:
+            attrib_obj.value = new_value
+        except IntegrityError:
+            # this can happen if the cache was stale and the databse object is
+            # missing. If so we need to clean self.hashid from the cache
+            if _GA(self, "hashid") in _ATTRIBUTE_CACHE:
+                del _ATTRIBUTE_CACHE[_GA(self, "hashid")]
+                self.delete()
+            raise IntegrityError("Attribute could not be saved - object %s was deleted from database." % self.key)
         _ATTRIBUTE_CACHE[_GA(self, "hashid")][attribute_name] = attrib_obj
 
     def get_attribute_obj(self, attribute_name, default=None):