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.

3 files changed, 1 insertions, 23 deletions

diff --git a/activerecord/lib/active_record/associations/belongs_to_association.rb b/activerecord/lib/active_record/associations/belongs_to_association.rb
index 3d4ad1dd5b..544aec5e8b 100644
--- a/activerecord/lib/active_record/associations/belongs_to_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_association.rb
@@ -50,11 +50,8 @@ module ActiveRecord
         def replace(record)
           if record
             raise_on_type_mismatch!(record)
-            update_counters_on_replace(record)
             set_inverse_instance(record)
             @updated = true
-          else
-            decrement_counters
           end
 
           replace_keys(record)
@@ -80,19 +77,6 @@ module ActiveRecord
           reflection.counter_cache_column && owner.persisted?
         end
 
-        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, touch: reflection.options[:touch])
-            decrement_counters
-          end
-        end
-
-        # Checks whether record is different to the current target, without loading it
-        def different_target?(record)
-          record._read_attribute(primary_key(record)) != owner._read_attribute(reflection.foreign_key)
-        end
-
         def replace_keys(record)
           owner[reflection.foreign_key] = record ? record._read_attribute(primary_key(record)) : nil
         end
diff --git a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
index 3fd2fb5f67..9ae452e7a1 100644
--- a/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_polymorphic_association.rb
@@ -19,10 +19,6 @@ module ActiveRecord
           owner[reflection.foreign_type] = record ? record.class.polymorphic_name : nil
         end
 
-        def different_target?(record)
-          super || record.class != klass
-        end
-
         def inverse_reflection_for(record)
           reflection.polymorphic_inverse_of(record.class)
         end
diff --git a/activerecord/lib/active_record/associations/builder/belongs_to.rb b/activerecord/lib/active_record/associations/builder/belongs_to.rb
index 0166ed98ca..da4cc343eb 100644
--- a/activerecord/lib/active_record/associations/builder/belongs_to.rb
+++ b/activerecord/lib/active_record/associations/builder/belongs_to.rb
@@ -34,9 +34,7 @@ module ActiveRecord::Associations::Builder # :nodoc:
           foreign_key  = reflection.foreign_key
           cache_column = reflection.counter_cache_column
 
-          if @_after_replace_counter_called ||= false
-            @_after_replace_counter_called = false
-          elsif association(reflection.name).target_changed?
+          if association(reflection.name).target_changed?
             if reflection.polymorphic?
               model     = attribute_in_database(reflection.foreign_type).try(:constantize)
               model_was = attribute_before_last_save(reflection.foreign_type).try(:constantize)