Allow primary_key: association option to be composite

Association's `primary_key` can be composite when derived from associated class
`primary_key` or `query_constraints`. But we don't allow setting it explicitly
even though Rails is already capable of supporting it.

This commit allows `primary_key` association option to be an array.

activerecord/lib/active_record/reflection.rb

@@ -870,7 +870,11 @@ def association_class
       # klass option is necessary to support loading polymorphic associations
       def association_primary_key(klass = nil)
         if primary_key = options[:primary_key]
-          @association_primary_key ||= -primary_key.to_s
+          @association_primary_key ||= if primary_key.is_a?(Array)
+            primary_key.map { |pk| pk.to_s.freeze }.freeze
+          else
+            -primary_key.to_s
+          end
         elsif (klass || self.klass).has_query_constraints? || options[:query_constraints]
           (klass || self.klass).composite_query_constraints_list
         elsif (klass || self.klass).composite_primary_key?

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -371,6 +371,18 @@ def test_building_the_belonging_object_for_composite_primary_key
     assert_equal id, cpk_book.order_id
   end
 
+  def test_belongs_to_with_explicit_composite_primary_key
+    cpk_book = cpk_books(:cpk_great_author_first_book)
+    order = cpk_book.build_order_explicit_fk_pk
+    order.shop_id = 123
+    cpk_book.save
+
+    shop_id, id = order.id
+    assert_equal id, cpk_book.order_id
+    assert_equal shop_id, cpk_book.shop_id
+    assert_equal order, cpk_book.reload.order_explicit_fk_pk
+  end
+
   def test_belongs_to_with_inverse_association_for_composite_primary_key
     author = Cpk::Author.new(name: "John")
     book = author.books.build(id: [nil, 1], title: "The Rails Way")

activerecord/test/models/cpk/book.rb

@@ -6,6 +6,7 @@ class Book < ActiveRecord::Base
 
     self.table_name = :cpk_books
     belongs_to :order, autosave: true, query_constraints: [:shop_id, :order_id], counter_cache: true
+    belongs_to :order_explicit_fk_pk, class_name: "Cpk::Order", query_constraints: [:shop_id, :order_id], primary_key: [:shop_id, :id]
     belongs_to :author, class_name: "Cpk::Author"
 
     has_many :chapters, query_constraints: [:author_id, :book_id]