Fix replacing foreign key for a CPK association by id attribute

Given a model associated with a composite primary key by `id` attribute,
for example:
```ruby
Order.primary_key = [:shop_id, :id]
OrderAgreement.primary_key = :id

OrderAgreement.belongs_to :order, primary_key: :id
```

Assigning `order` to an `OrderAgreement` object will replace the
`order_id` foreign key:

```ruby
order = Order.last # => #<Order id: 1, shop_id: 2>

order_agreement = OrderAgreement.new(order: order).save
order_agreement.order_id # => 1
```

activerecord/lib/active_record/autosave_association.rb

@@ -505,12 +505,12 @@ def save_belongs_to_association(reflection)
             saved = record.save(validate: !autosave) if record.new_record? || (autosave && record.changed_for_autosave?)
 
             if association.updated?
-              primary_key = Array(compute_primary_key(reflection, record))
+              primary_key = Array(compute_primary_key(reflection, record)).map(&:to_s)
               foreign_key = Array(reflection.foreign_key)
 
               primary_key_foreign_key_pairs = primary_key.zip(foreign_key)
               primary_key_foreign_key_pairs.each do |primary_key, foreign_key|
-                association_id = record.public_send(primary_key)
+                association_id = record._read_attribute(primary_key)
                 self[foreign_key] = association_id unless self[foreign_key] == association_id
               end
               association.loaded!
@@ -527,7 +527,7 @@ def compute_primary_key(reflection, record)
         elsif reflection.options[:query_constraints] && (query_constraints = record.class.query_constraints_list)
           query_constraints
         else
-          :id
+          record.class.primary_key
         end
       end
 

activerecord/test/cases/associations_test.rb

@@ -287,6 +287,22 @@ def test_assign_composite_foreign_key_belongs_to_association
     assert_equal(another_blog.id, comment.blog_id)
   end
 
+
+  def test_assign_belongs_to_cpk_model_by_id_attribute
+    order = cpk_orders(:cpk_groceries_order_1)
+    agreement = Cpk::OrderAgreement.new(signature: "signed")
+
+    agreement.order = order
+    agreement.save
+
+    assert_not_nil(agreement.reload.order)
+    assert_not_nil(agreement.order_id)
+
+    assert_equal(order, agreement.order)
+    _shop_id, order_id = order.id
+    assert_equal(order_id, agreement.order_id)
+  end
+
   def test_append_composite_foreign_key_has_many_association_with_autosave
     blog_post = sharded_blog_posts(:great_post_blog_one)
     comment = Sharded::Comment.new(body: "Great post! :clap:")