Fix #scope_for_create for belongs_to associations with CPK

`#scope_for_create` for singular associations removes the primary key
from the scope so that we don't assign PK columns when building an
association.

However, removing the primary key from the scope doesn't currently
handle composite primary keys. This commit fixes that.

Author: adrianna-chang-shopify

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