Fix `belongs_to_counter_cache_after_update` to respect custom primary key counter

If belongs_to primary key is customized, the callback will update
counters against the wrong target looked up by the customized key as
primary key.
We need to convert the customized key into an object that can be
referred to as primary key.

2 files changed, 20 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index c161454c1a..852a858d9b 100644
--- a/activerecord/lib/active_record/associations/builder/belongs_to.rb
+++ b/activerecord/lib/active_record/associations/builder/belongs_to.rb
@@ -49,14 +49,27 @@ module ActiveRecord::Associations::Builder # :nodoc:
             foreign_key     = attribute_in_database foreign_key
 
             if foreign_key && model.respond_to?(:increment_counter)
+              foreign_key = counter_cache_target(reflection, model, foreign_key)
               model.increment_counter(cache_column, foreign_key)
             end
 
             if foreign_key_was && model_was.respond_to?(:decrement_counter)
+              foreign_key_was = counter_cache_target(reflection, model_was, foreign_key_was)
               model_was.decrement_counter(cache_column, foreign_key_was)
             end
           end
         end
+
+        private
+          def counter_cache_target(reflection, model, foreign_key)
+            primary_key = reflection.association_primary_key(model)
+
+            if primary_key == model.primary_key
+              foreign_key
+            else
+              model.unscoped.where!(primary_key => foreign_key)
+            end
+          end
       end
     end
 
diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index c0dfa91a21..856bc70f6b 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -466,7 +466,13 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
   def test_belongs_to_with_primary_key_counter
     debate  = Topic.create("title" => "debate")
     debate2 = Topic.create("title" => "debate2")
-    reply   = Reply.create("title" => "blah!", "content" => "world around!", "parent_title" => "debate")
+    reply   = Reply.create("title" => "blah!", "content" => "world around!", "parent_title" => "debate2")
+
+    assert_equal 0, debate.reload.replies_count
+    assert_equal 1, debate2.reload.replies_count
+
+    reply.parent_title = "debate"
+    reply.save!
 
     assert_equal 1, debate.reload.replies_count
     assert_equal 0, debate2.reload.replies_count