Don't expose `add_to_transaction`

`add_to_transaction` was added at da840d13, but it should not be called
by except internal, since `remember_transaction_record_state` should be
called only once before saving.

And also, currently `add_to_transaction` doesn't always add the record
to transaction since da8de911, that is the reason hard to use that even
in internal.

Even if `add_to_transaction` ensure to add the record to transaction,
that is an internal concern, people don't need to explicitly call
`add_to_transaction`.

activerecord/lib/active_record/touch_later.rb

@@ -22,7 +22,7 @@ def touch_later(*names) # :nodoc:
       @_touch_time = current_time_from_proper_timezone
 
       surreptitiously_touch @_defer_touch_attrs
-      self.class.connection.add_transaction_record self
+      add_to_transaction
 
       # touch the parents as we are not calling the after_save callbacks
       self.class.reflect_on_all_associations(:belongs_to).each do |r|

activerecord/lib/active_record/transactions.rb

@@ -355,18 +355,6 @@ def rolledback!(force_restore_state: false, should_run_callbacks: true) #:nodoc:
       clear_transaction_record_state
     end
 
-    # Add the record to the current transaction so that the #after_rollback and #after_commit callbacks
-    # can be called.
-    def add_to_transaction
-      if has_transactional_callbacks?
-        self.class.connection.add_transaction_record(self)
-      else
-        sync_with_transaction_state
-        set_transaction_state(self.class.connection.transaction_state)
-      end
-      remember_transaction_record_state
-    end
-
     # Executes +method+ within a transaction and captures its return value as a
     # status flag. If the status is true the transaction is committed, otherwise
     # a ROLLBACK is issued. In any case the status flag is returned.
@@ -387,7 +375,7 @@ def with_transaction_returning_status
       ensure
         if has_transactional_callbacks? &&
             (@_new_record_before_last_commit && !new_record? || _trigger_update_callback || _trigger_destroy_callback)
-          self.class.connection.add_transaction_record(self)
+          add_to_transaction
         end
       end
       status
@@ -459,6 +447,12 @@ def transaction_include_any_action?(actions)
         end
       end
 
+      # Add the record to the current transaction so that the #after_rollback and #after_commit
+      # callbacks can be called.
+      def add_to_transaction
+        self.class.connection.add_transaction_record(self)
+      end
+
       def set_transaction_state(state)
         @transaction_state = state
       end

activerecord/test/cases/transaction_callbacks_test.rb

@@ -624,7 +624,7 @@ def test_commit_run_transactions_callbacks_with_explicit_enrollment
         @topic.content = "foo"
         @topic.save!
       end
-      @topic.class.connection.add_transaction_record(@topic)
+      @topic.send(:add_to_transaction)
     end
     assert_equal [:before_commit, :after_commit], @topic.history
   end
@@ -634,7 +634,7 @@ def test_commit_run_transactions_callbacks_with_nested_transactions
       @topic.transaction(requires_new: true) do
         @topic.content = "foo"
         @topic.save!
-        @topic.class.connection.add_transaction_record(@topic)
+        @topic.send(:add_to_transaction)
       end
     end
     assert_equal [:before_commit, :after_commit], @topic.history
@@ -655,7 +655,7 @@ def test_rollback_run_transactions_callbacks_with_explicit_enrollment
         @topic.content = "foo"
         @topic.save!
       end
-      @topic.class.connection.add_transaction_record(@topic)
+      @topic.send(:add_to_transaction)
       raise ActiveRecord::Rollback
     end
     assert_equal [:rollback], @topic.history

activerecord/test/cases/transactions_test.rb

@@ -65,7 +65,7 @@ def transaction_with_return
 
   def test_add_to_null_transaction
     topic = Topic.new
-    topic.add_to_transaction
+    topic.send(:add_to_transaction)
   end
 
   def test_successful_with_return