Merge pull request rails#48400 from adrianna-chang-shopify/ac-scope-for-create-on-cpk

Fix `#scope_for_create` for belongs_to associations with CPK

Author: eileencodes

activerecord/lib/active_record/associations/singular_association.rb

@@ -34,7 +34,7 @@ def force_reload_reader
 
       private
         def scope_for_create
-          super.except!(klass.primary_key)
+          super.except!(*Array(klass.primary_key))
         end
 
         def find_target

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -35,7 +35,7 @@
 class BelongsToAssociationsTest < ActiveRecord::TestCase
   fixtures :accounts, :companies, :developers, :projects, :topics,
            :developers_projects, :computers, :authors, :author_addresses,
-           :essays, :posts, :tags, :taggings, :comments, :sponsors, :members, :nodes
+           :essays, :posts, :tags, :taggings, :comments, :sponsors, :members, :nodes, :cpk_books
 
   def test_belongs_to
     client = Client.find(3)
@@ -356,6 +356,15 @@ def test_building_the_belonging_object
     assert_equal apple.id, citibank.firm_id
   end
 
+  def test_building_the_belonging_object_for_composite_primary_key
+    cpk_book = cpk_books(:cpk_great_author_first_book)
+    order = cpk_book.build_order
+    cpk_book.save
+
+    _shop_id, id = order.id
+    assert_equal id, cpk_book.order_id
+  end
+
   def test_building_the_belonging_object_with_implicit_sti_base_class
     account = Account.new
     company = account.build_firm