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: 20220514222125_create_versions.rb
    path of file: ./repos/chordbook/db/migrate
    
    the code of the file until where you have to start completion: # This migration creates the `versions` table, the only schema PT requires.
# All other migrations PT provides are optional.
class CreateVersions < ActiveRecord::Migration[7.0]
  def change
    create_table :versions do |t|
      t.string :item_type, null: false
      t.bigint :item_id, null: false
      t.string :event, null: false
      t.string :whodunnit
      t.json :object

      # Known issue in MySQL: fractional second precision
      # -------------------------------------------------
      #
      # MySQL timestamp columns do not support fractional seconds unless
      # defined with "fractional seconds precision". MySQL users should manually
      # add fractional seconds precision to this migration, specifically, to
      # the `created_at` column.
      # (https://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html)
      #
      # MySQL users should also upgrade to 
    
  def change
    create_table :versions do |t|
      t.string :item_type, null: false
      t.bigint :item_id, null: false
      t.string :event, null: false
      t.string :whodunnit
      t.json :object

      # Known issue in MySQL: fractional second precision
      # -------------------------------------------------
      #
      # MySQL timestamp columns do not support fractional seconds unless
      # defined with "fractional seconds precision". MySQL users should manually
      # add fractional seconds precision to this migration, specifically, to
      # the `created_at` column.
      # (https://dev.mysql.com/doc/refman/5.6/en/fractional-seconds.html)
      #
      # MySQL users should also upgrade to at least rails 4.2, which is the first
      # version of ActiveRecord with support for fractional seconds in MySQL.
      # (https://github.com/rails/rails/pull/14359)
      #
      # MySQL users should use the following line for `created_at`
      # t.datetime :created_at, limit: 6
      t.datetime :created_at
    end
    add_index :versions, %i[item_type item_id]
  end
end