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

问题

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 is a newly created record or an old one from update?

I'm hoping not to use another callback such as before_create to set a flag in the model or require another query into the db.

Any advice is appreciated.

Edit: Need to determine it in after_save hook, and for my particular use case, there is no updated_at or updated_on timestamp

回答1:

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.

回答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

回答3:

Yet another option, for those who do have an updated_at timestamp:

if created_at == updated_at
  # it's a newly created record
end

回答4:

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

回答5:

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!

来源：https://stackoverflow.com/questions/4975106/how-to-determine-if-a-record-is-just-created-or-updated-in-after-save

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ruby-on-rails

activerecord

activemodel