Fix inconsistent touching behavior between assigning and unassigning

On belongs_to with `touch: true` association, unassigned object is
caused touching, but assigned object is not touched.

And also, if primary key is customized, it will touch against the wrong
target looked up by the customized key as primary key.

This change ensures correctly touching consistently between assigning
and unassigning.

4 files changed, 29 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/associations/belongs_to_association.rb b/activerecord/lib/active_record/associations/belongs_to_association.rb
index cea2570ac4..08f450278d 100644
--- a/activerecord/lib/active_record/associations/belongs_to_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_association.rb
@@ -83,7 +83,7 @@ module ActiveRecord
         def update_counters_on_replace(record)
           if require_counter_update? && different_target?(record)
             owner.instance_variable_set :@_after_replace_counter_called, true
-            record.increment!(reflection.counter_cache_column)
+            record.increment!(reflection.counter_cache_column, touch: reflection.options[:touch])
             decrement_counters
           end
         end
diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index 9e012285e5..4b6cb76081 100644
--- a/activerecord/lib/active_record/associations/builder/belongs_to.rb
+++ b/activerecord/lib/active_record/associations/builder/belongs_to.rb
@@ -92,7 +92,8 @@ module ActiveRecord::Associations::Builder # :nodoc:
         else
           klass = association.klass
         end
-        old_record = klass.find_by(klass.primary_key => old_foreign_id)
+        primary_key = reflection.association_primary_key(klass)
+        old_record = klass.find_by(primary_key => old_foreign_id)
 
         if old_record
           if touch != true
diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index 6cbde2796a..9ca9ab62a1 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -557,6 +557,31 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert_equal 1, Topic.find(topic.id)[:replies_count]
   end
 
+  def test_belongs_to_touch_with_reassigning
+    debate  = Topic.create!(title: "debate")
+    debate2 = Topic.create!(title: "debate2")
+    reply   = Reply.create!(title: "blah!", content: "world around!", parent_title: "debate2")
+
+    time = 1.day.ago
+
+    debate.touch(time: time)
+    debate2.touch(time: time)
+
+    reply.parent_title = "debate"
+    reply.save!
+
+    assert_operator debate.reload.updated_at, :>, time
+    assert_operator debate2.reload.updated_at, :>, time
+
+    debate.touch(time: time)
+    debate2.touch(time: time)
+
+    reply.topic_with_primary_key = debate2
+
+    assert_operator debate.reload.updated_at, :>, time
+    assert_operator debate2.reload.updated_at, :>, time
+  end
+
   def test_belongs_to_with_touch_option_on_touch
     line_item = LineItem.create!
     Invoice.create!(line_items: [line_item])
diff --git a/activerecord/test/models/reply.rb b/activerecord/test/models/reply.rb
index bc829ec67f..d1cee58788 100644
--- a/activerecord/test/models/reply.rb
+++ b/activerecord/test/models/reply.rb
@@ -4,7 +4,7 @@ require "models/topic"
 
 class Reply < Topic
   belongs_to :topic, foreign_key: "parent_id", counter_cache: true
-  belongs_to :topic_with_primary_key, class_name: "Topic", primary_key: "title", foreign_key: "parent_title", counter_cache: "replies_count"
+  belongs_to :topic_with_primary_key, class_name: "Topic", primary_key: "title", foreign_key: "parent_title", counter_cache: "replies_count", touch: true
   has_many :replies, class_name: "SillyReply", dependent: :destroy, foreign_key: "parent_id"
 end