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: relationship.rb
    path of file: ./repos/puppet/lib/puppet
    
    the code of the file until where you have to start completion: # frozen_string_lit
    
  def to_data_hash
    data = {
      'source' => source.to_s,
      'target' => target.to_s
    }
    data['event'] = event.to_s unless event.nil?
    data['callback'] = callback.to_s unless callback.nil?
    data
  end