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: required_matcher.rb
    path of file: ./repos/shoulda-matchers/lib/shoulda/matchers/active_record/association_matchers
    
    the code of the file until where you have to start completion: module Shoulda
  module Matchers
    module ActiveRecord
      module AssociationMatchers
        # @private
        class RequiredMatcher
          attr_reader :missing_option

          def initial
    
          def initialize(attribute_name, required)
            @attribute_name = attribute_name
            @required = required
            @submatcher = ActiveModel::DisallowValueMatcher.new(nil).
              for(attribute_name).
              with_message(validation_message_key)
            @missing_option = ''
          end