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: association.rb
    path of file: ./repos/rails_admin/lib/rails_admin/adapters/composite_primary_keys
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

module RailsAdmin
  module Adapters
    module CompositePrimaryKeys
      class Assoc
    
        def primary_key
          return nil if polymorphic?

          value = association.association_primary_key

          if value.is_a? Array
            :id
          else
            value.to_sym
          end