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: 20180710172959_disallow_multi_levels_theme_components.rb
    path of file: ./repos/discourse/db/migrate
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

class DisallowMultiL
    
  def already_exists?(parent, child)
    DB.query(
      "SELECT 1 AS one FROM child_themes WHERE child_theme_id = :child AND parent_theme_id = :parent",
      child: child,
      parent: parent,
    ).present?
  end