diff --git a/lib/geocoder/stores/mongoid.rb b/lib/geocoder/stores/mongoid.rb
index ca5841366909f985aa44cf30fd3e44889259f872..ff19d40c746f01850bfa433cfc538e8dedb53273 100644
--- a/lib/geocoder/stores/mongoid.rb
+++ b/lib/geocoder/stores/mongoid.rb
@@ -8,6 +8,16 @@ module Geocoder::Store
 
     def self.included(base)
       MongoBase.included_by_model(base)
+      base.class_eval do
+
+        ##
+        # Name of the attribute to use when determining whether two
+        # records are the same record.
+        #
+        def self.primary_key
+          "_id"
+        end
+      end
     end
   end
 end