just do it in the controller

app/controllers/mappings_controller.rb

@@ -45,6 +45,13 @@ class MappingsController < ApplicationController
     @mapping = Mapping.find(params[:id])
     authorize @mapping
 
+    mappable = @mapping.mappable
+    if mappable.defer_to_map
+      mappable.permission = mappable.defer_to_map.permission
+      mappable.defer_to_map_id = nil
+      mappable.save
+    end
+
     @mapping.destroy
 
     head :no_content 

app/models/mapping.rb

@@ -7,8 +7,6 @@ class Mapping < ActiveRecord::Base
   belongs_to :map, :class_name => "Map", :foreign_key => "map_id", touch: true
   belongs_to :user
 
-  before_destroy :remove_defer
-
   validates :xloc, presence: true, 
     unless: Proc.new { |m| m.mappable_type == 'Synapse' }
   validates :yloc, presence: true,
@@ -28,11 +26,4 @@ class Mapping < ActiveRecord::Base
     super(:methods =>[:user_name, :user_image])
   end
 
-  private
-
-    def remove_defer(mapping)
-      mappable = mapping.mappable
-      mappable.defer_to_map_id = nil
-      mappable.save
-    end
 end