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: smart_proxy_host_extensions.rb
    path of file: ./repos/foreman/app/models/concerns
    
    the code of the file until where you have to start completion: module SmartProxyHostExten
    
    def proxy_column_list
      columns = proxy_reference_groups_by_table.map do |table_name, reference_group|
        reference_group.flat_map.with_index do |ref, idx|
          ref.map_column_names idx
        end
      end
      columns.flatten.concat(proxy_self_columns).join(',')
    end