Fix issue with IDs reader on preloaded associations for composite primary 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*
+
 *   Allow to configure `strict_loading_mode` globally or within a model.
 
     Defaults to `:all`, can be changed to `:n_plus_one_only`.

activerecord/lib/active_record/associations/collection_association.rb

@@ -50,11 +50,11 @@ module ActiveRecord
       # 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

@@ -3234,6 +3234,12 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     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