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: 20181209000000_create_activity_notification_tables.rb
    path of file: ./repos/activity_notification/spec/rails_app/db/migrate
    
    the code of the file until where you have to start completion: # Migration responsible for creating a table with notifications
class CreateActivityNotificationTables < ActiveRecord::Migration[5.2]
  # Create tables
  def change
    create_table :notifications do |t|
      t.belongs_to :target,     polymorphic: true, index: true, null: false
      t.belongs_to :notifiable, polymorphic: true, index: true, null: false
      t.string     :key,                                        null: false
      t.belongs_to :group,      polymorphic: true, index: true
      t.integer    :group_owner_id,                index: true
      t.belon
    
  def change
    create_table :notifications do |t|
      t.belongs_to :target,     polymorphic: true, index: true, null: false
      t.belongs_to :notifiable, polymorphic: true, index: true, null: false
      t.string     :key,                                        null: false
      t.belongs_to :group,      polymorphic: true, index: true
      t.integer    :group_owner_id,                index: true
      t.belongs_to :notifier,   polymorphic: true, index: true
      t.text       :parameters
      t.datetime   :opened_at

      t.timestamps null: false
    end

    create_table :subscriptions do |t|
      t.belongs_to :target,     polymorphic: true, index: true, null: false
      t.string     :key,                           index: true, null: false
      t.boolean    :subscribing,                                null: false, default: true
      t.boolean    :subscribing_to_email,                       null: false, default: true
      t.datetime   :subscribed_at
      t.datetime   :unsubscribed_at
      t.datetime   :subscribed_to_email_at
      t.datetime   :unsubscribed_to_email_at
      t.text       :optional_targets

      t.timestamps null: false
    end
    add_index :subscriptions, [:target_type, :target_id, :key], unique: true
  end