Run a callback only if an attribute has changed in Rails

Question

I have the following association in my app:

# Page 
belongs_to :status

I want to run a callback anytime the status_id of a page has changed.

So, if page.status_id goes from 4 to 5, I want to be able to catch that.

How to do so?

Answer 1

Rails 5.1+

class Page < ActiveRecord::Base
  before_save :do_something, if: :will_save_change_to_status_id?

  private

  def do_something
    # ...
  end
end

The commit that changed ActiveRecord::Dirty is here: https://github.com/rails/rails/commit/16ae3db5a5c6a08383b974ae6c96faac5b4a3c81

Here is a blog post on these changes: https://www.ombulabs.com/blog/rails/upgrades/active-record-5-1-api-changes.html

Here is the summary I made for myself on the changes to ActiveRecord::Dirty in Rails 5.1+:

ActiveRecord::Dirty

https://api.rubyonrails.org/classes/ActiveRecord/AttributeMethods/Dirty.html

Before Saving (OPTIONAL CHANGE)

After modifying an object and before saving to the database, or within the before_save filter:

• changes should now be changes_to_save
• changed? should now be has_changes_to_save?
• changed should now be changed_attribute_names_to_save
• <attribute>_change should now be <attribute>_change_to_be_saved
• <attribute>_changed? should now be will_save_change_to_<attribute>?
• <attribute>_was should now be <attribute>_in_database

After Saving (BREAKING CHANGE)

After modifying an object and after saving to the database, or within the after_save filter:

• saved_changes (replaces previous_changes)
• saved_changes?
• saved_change_to_<attribute>
• saved_change_to_<attribute>?
• <attribute>_before_last_save

---

Rails <= 5.0

class Page < ActiveRecord::Base
  before_save :do_something, if: :status_id_changed?

  private

  def do_something
    # ...
  end
end

This utilizes the fact that the before_save callback can conditionally execute based on the return value of a method call. The status_id_changed? method comes from ActiveModel::Dirty, which allows us to check if a specific attribute has changed by simply appending _changed? to the attribute name.

When the do_something method should be called is up to your needs. It could be before_save or after_save or any of the defined ActiveRecord::Callbacks.