Remove and ignore notes.original_discussion_id column

app/models/concerns/ignorable_column.rb

+# Module that can be included into a model to make it easier to ignore database
+# columns.
+#
+# Example:
+#
+#     class User < ActiveRecord::Base
+#       include IgnorableColumn
+#
+#       ignore_column :updated_at
+#     end
+#
+module IgnorableColumn
+  extend ActiveSupport::Concern
+
+  module ClassMethods
+    def columns
+      super.reject { |column| ignored_columns.include?(column.name) }
+    end
+
+    def ignored_columns
+      @ignored_columns ||= Set.new
+    end
+
+    def ignore_column(name)
+      ignored_columns << name.to_s
+    end
+  end
+end

app/models/note.rb

@@ -9,6 +9,9 @@ class Note < ActiveRecord::Base
   include CacheMarkdownField
   include AfterCommitQueue
   include ResolvableNote
+  include IgnorableColumn
+
+  ignore_column :original_discussion_id
 
   cache_markdown_field :note, pipeline: :note
 

db/post_migrate/20170404170532_remove_notes_original_discussion_id.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class RemoveNotesOriginalDiscussionId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  # When using the methods "add_concurrent_index" or "add_column_with_default"
+  # you must disable the use of transactions as these methods can not run in an
+  # existing transaction. When using "add_concurrent_index" make sure that this
+  # method is the _only_ method called in the migration, any other changes
+  # should go in a separate migration. This ensures that upon failure _only_ the
+  # index creation fails and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+  # disable_ddl_transaction!
+
+  def change
+    remove_column :notes, :original_discussion_id, :string
+  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: 20170402231018) do
+ActiveRecord::Schema.define(version: 20170404170532) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -724,7 +724,6 @@ ActiveRecord::Schema.define(version: 20170402231018) do
     t.datetime "resolved_at"
     t.integer "resolved_by_id"
     t.string "discussion_id"
-    t.string "original_discussion_id"
     t.text "note_html"
   end