How to determine if a record is just created or updated in after_save

Question

The #new_record? function determines if a record has been saved. But it is always false in the after_save hook. Is there a way to determine whether the record i

Answer 1

For Rails 4 (checked on 4.2.11.1) results of changes and previous_changes methods are empty hashes {} on object creation inside after_save. So attribute_changed? methods like id_changed? won't work as expected.

But you can take advantage of this knowledge and - knowing that at least 1 attribute has to be in changes on update - check if changes is empty. Once you confirm that it's empty, you must be during object creation:

after_save do
  if changes.empty?
    # code appropriate for object creation goes here ...
  end
end

Answer 2

No rails magic here that I know of, you'll have to do it yourself. You could clean this up using a virtual attribute...

In your model class:

def before_save
  @was_a_new_record = new_record?
  return true
end

def after_save
  if @was_a_new_record
    ...
  end
end

Answer 3

I was looking to use this for an after_save callback.

A simpler solution is to use id_changed? (since it won't change on update) or even created_at_changed? if timestamp columns are present.

Update: As @mitsy points out, if this check is needed outside of callbacks then use id_previously_changed?. See docs.

Answer 4

Since the object has already been saved, you would you need to look at the previous changes. The ID should only change after a create.

# true if this is a new record
@object.previous_changes[:id].any?

There is also an instance variable @new_record_before_save. You can access that by doing the following:

# true if this is a new record
@object.instance_variable_get(:@new_record_before_save)

Both are pretty ugly, but they would allow you to know whether the object has been newly created. Hope that helps!

Answer 5

There is an after_create callback which is only called if the record is a new record, after it is saved. There is also an after_update callback for use if this was an existing record which was changed and saved. The after_save callback is called in both cases, after either after_create or after_update is called.

Use after_create if you need something to happen once after a new record has been saved.

More info here: http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html

Answer 6

I like to be specific even I aware that :id shouldn't change in normal, but

(byebug) id_change.first.nil?
true

It always cheaper to be specific instead finding very weird unexpected bug.

The same way if I expect true flag from untrustworthy argument

def foo?(flag)
  flag == true
end

This saves a lot of hours to not sitting on weird bugs.