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: 20150317224428_acts_as_taggable_on_migration.acts_as_taggable_on_engine.rb
    path of file: ./repos/call4paperz/db/migrate
    
    the code of the file until where you have to start completion: # This migration comes from acts_as_taggable_on_engine (originall
    
  def self.up
    create_table :tags do |t|
      t.string :name
    end

    create_table :taggings do |t|
      t.references :tag

      # You should make sure that the column created is
      # long enough to store the required class names.
      t.references :taggable, polymorphic: true
      t.references :tagger, polymorphic: true

      # Limit is created to prevent MySQL error on index
      # length for MyISAM table type: http://bit.ly/vgW2Ql
      t.string :context, limit: 128

      t.datetime :created_at
    end

    add_index :taggings, [:taggable_id, :taggable_type, :context]
  end