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, 6 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 477bc1b54a..f79ed01122 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Don't update counter cache unless the record is actually saved.
+
+    Fixes #31493, #33113, #33117.
+
+    *Ryuta Kamizono*
+
 *   SQLite3 adapter supports expression indexes.
 
     ```