You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 20170809182148_add_missing_indexes_on_taggings.acts_as_taggable_on_engine.rb
    path of file: ./repos/forem/db/migrate
    
    the code of the file until where you have to start completion: # This migration comes from acts_as_taggable_on_engine (originally 6)
if ActiveRecord.gem_version >= Gem::Version.new('5.0')
  class AddMissingIndexesOnTaggings < ActiveRecord::Migration[4.2]; end
else
  class AddMissingIndexesOnTaggings < ActiveRecord::Mi
    
  def change
    add_index :taggings, :tag_id unless index_exists? :taggings, :tag_id
    add_index :taggings, :taggable_id unless index_exists? :taggings, :taggable_id
    add_index :taggings, :taggable_type unless index_exists? :taggings, :taggable_type
    add_index :taggings, :tagger_id unless index_exists? :taggings, :tagger_id
    add_index :taggings, :context unless index_exists? :taggings, :context

    unless index_exists? :taggings, [:tagger_id, :tagger_type]
      add_index :taggings, [:tagger_id, :tagger_type]
    end