Don't update counter cache unless the record is actually saved

This is a 4th attempt to make counter cache transactional completely.

Past attempts: #9236, #14849, #23357.

All existing counter cache issues (increment/decrement twice, lost
increment) are caused due to updating counter cache on the outside of
the record saving transaction by assigning belongs_to record, even
though assigning that doesn't cause the record saving.

We have the `@_after_replace_counter_called` guard condition to mitigate
double increment/decrement issues, but we can't completely prevent that
inconsistency as long as updating counter cache on the outside of the
transaction, since saving the record is not always happened after that.

We already have handling counter cache after create/update/destroy,

https://github.com/rails/rails/blob/1b90f614b1b3d06b7f02a8b9ea6cd84f15d58643/activerecord/lib/active_record/counter_cache.rb#L162-L189
https://github.com/rails/rails/blob/1b90f614b1b3d06b7f02a8b9ea6cd84f15d58643/activerecord/lib/active_record/associations/builder/belongs_to.rb#L33-L59

so just removing assigning logic on the belongs_to association makes
counter cache transactional completely.

Closes #14849.
Closes #23357.
Closes #31493.
Closes #31494.
Closes #32372.
Closes #33113.
Closes #33117
Closes #33129.
Closes #33458.

1 files changed, 36 insertions, 6 deletions

diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index 169ff96b94..43763dc715 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -452,15 +452,39 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
   end
 
   def test_belongs_to_counter_with_assigning_nil
-    post = Post.find(1)
-    comment = Comment.find(1)
+    topic = Topic.create!(title: "debate")
+    reply = Reply.create!(title: "blah!", content: "world around!", topic: topic)
 
-    assert_equal post.id, comment.post_id
-    assert_equal 2, Post.find(post.id).comments.size
+    assert_equal topic.id, reply.parent_id
+    assert_equal 1, topic.reload.replies.size
 
-    comment.post = nil
+    reply.topic = nil
+    reply.reload
 
-    assert_equal 1, Post.find(post.id).comments.size
+    assert_equal topic.id, reply.parent_id
+    assert_equal 1, topic.reload.replies.size
+
+    reply.topic = nil
+    reply.save!
+
+    assert_equal 0, topic.reload.replies.size
+  end
+
+  def test_belongs_to_counter_with_assigning_new_object
+    topic = Topic.create!(title: "debate")
+    reply = Reply.create!(title: "blah!", content: "world around!", topic: topic)
+
+    assert_equal topic.id, reply.parent_id
+    assert_equal 1, topic.reload.replies_count
+
+    topic2 = reply.build_topic(title: "debate2")
+    reply.save!
+
+    assert_not_equal topic.id, reply.parent_id
+    assert_equal topic2.id, reply.parent_id
+
+    assert_equal 0, topic.reload.replies_count
+    assert_equal 1, topic2.reload.replies_count
   end
 
   def test_belongs_to_with_primary_key_counter
@@ -485,11 +509,13 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert_equal 0, debate2.reload.replies_count
 
     reply.topic_with_primary_key = debate2
+    reply.save!
 
     assert_equal 0, debate.reload.replies_count
     assert_equal 1, debate2.reload.replies_count
 
     reply.topic_with_primary_key = nil
+    reply.save!
 
     assert_equal 0, debate.reload.replies_count
     assert_equal 0, debate2.reload.replies_count
@@ -516,11 +542,13 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert_equal 1, Topic.find(topic2.id).replies.size
 
     reply1.topic = nil
+    reply1.save!
 
     assert_equal 0, Topic.find(topic1.id).replies.size
     assert_equal 0, Topic.find(topic2.id).replies.size
 
     reply1.topic = topic1
+    reply1.save!
 
     assert_equal 1, Topic.find(topic1.id).replies.size
     assert_equal 0, Topic.find(topic2.id).replies.size
@@ -594,6 +622,7 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     debate2.touch(time: time)
 
     reply.topic_with_primary_key = debate2
+    reply.save!
 
     assert_operator debate.reload.updated_at, :>, time
     assert_operator debate2.reload.updated_at, :>, time
@@ -772,6 +801,7 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
 
     reply = Reply.create(title: "re: zoom", content: "speedy quick!")
     reply.topic = topic
+    reply.save!
 
     assert_equal 1, topic.reload[:replies_count]
     assert_equal 1, topic.replies.size