Remove all memberships where belonging group was deleted.

db/migrate/20140921104907_remove_stale_memberships.rb

@@ -0,0 +1,5 @@
+class RemoveStaleMemberships < ActiveRecord::Migration
+  def up
+    Membership.where("group_id NOT IN (?)", Group.ids).delete_all
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20140521142651) do
+ActiveRecord::Schema.define(version: 20140921104907) do
 
   create_table "article_categories", force: true do |t|
     t.string "name",        default: "", null: false