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: 20150317203743_change_project_id_to_not_null_in_unsubscribes.rb
    path of file: ./repos/catarse/db/migrate
    
    the code of the file until where you have to start completion: class ChangeProjectIdToNotN
    
  def change
    execute "
    UPDATE users SET subscribed_to_project_posts = false
    WHERE EXISTS (SELECT true FROM unsubscribes u WHERE u.user_id = users.id AND u.project_id IS NULL);
    DELETE FROM unsubscribes WHERE project_id IS NULL;
    "
    change_column_null :unsubscribes, :project_id, false
  end