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: 20210817152123_create_lessons_board_lessons.rb
    path of file: ./repos/i-diario/db/migrate
    
    the code of the file until where you have to start completion: class CreateLessonsBoardLessons < ActiveRecord::Migration[4.
    
  def change
    create_table :lessons_board_lessons do |t|
      t.integer :lessons_board_id
      t.string :lesson_number
      t.timestamps
      t.datetime :discarded_at
    end
    add_index :lessons_board_lessons, :lessons_board_id
    add_foreign_key :lessons_board_lessons, :lessons_boards, column: :lessons_board_id
  end