Fix issue with IDs reader on preloaded associations for composite pri…

…mary keys

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Fix an issue where the IDs reader method did not return expected results for preloaded associations
+    in models using composite primary keys.
+
+    *Jay Ang*
+
 *   Raise an `ActiveRecord::ActiveRecordError` error when the MySQL database returns an invalid version string.
 
     *Kevin McPhillips*

activerecord/lib/active_record/associations/collection_association.rb

@@ -48,11 +48,11 @@ def writer(records)
       # Implements the ids reader method, e.g. foo.item_ids for Foo.has_many :items
       def ids_reader
         if loaded?
-          target.pluck(reflection.association_primary_key)
+          target.pluck(*reflection.association_primary_key)
         elsif !target.empty?
-          load_target.pluck(reflection.association_primary_key)
+          load_target.pluck(*reflection.association_primary_key)
         else
-          @association_ids ||= scope.pluck(reflection.association_primary_key)
+          @association_ids ||= scope.pluck(*reflection.association_primary_key)
         end
       end
 

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -3232,6 +3232,12 @@ def test_key_ensuring_owner_was_is_valid_when_dependent_option_is_destroy_async
     MESSAGE
   end
 
+  def test_ids_reader_on_preloaded_association_with_composite_primary_key
+    great_author = cpk_authors(:cpk_great_author)
+
+    assert_equal great_author.books.ids, Cpk::Author.preload(:books).find(great_author.id).book_ids
+  end
+
   private
     def force_signal37_to_load_all_clients_of_firm
       companies(:first_firm).clients_of_firm.load_target