Fix document count issue adding a hint

Author: WaVEV

django_mongodb_backend/cache.py

@@ -89,7 +89,7 @@ def get_many(self, keys, version=None):
     def set(self, key, value, timeout=DEFAULT_TIMEOUT, version=None):
         key = self.make_and_validate_key(key, version=version)
         serialized_data = self.serializer.dumps(value)
-        num = self.collection_to_write.count_documents({})
+        num = self.collection_to_write.count_documents({}, hint="_id_")
         if num >= self._max_entries:
             self._cull(num)
         return self.collection_to_write.update_one(
@@ -107,7 +107,7 @@ def set(self, key, value, timeout=DEFAULT_TIMEOUT, version=None):
     def add(self, key, value, timeout=DEFAULT_TIMEOUT, version=None):
         key = self.make_and_validate_key(key, version=version)
         serialized_data = self.serializer.dumps(value)
-        num = self.collection_to_write.count_documents({})
+        num = self.collection_to_write.count_documents({}, hint="_id_")
         if num >= self._max_entries:
             self._cull(num)
         try: