Add virtual attribute to json output

The serialization of objects in Rails has two steps:

• First, as_json is called to convert the object to a simplified Hash.
• Then, to_json is called on the as_json return value to get the final JSON string.

You generally want to leave to_json alone so all you need to do is add your own as_json implementation sort of like this:

def as_json(options = { })
  # just in case someone says as_json(nil) and bypasses
  # our default...
  super((options || { }).merge({
    :methods => [:finished_items, :unfinished_items]
  }))
end

You could also do it like this:

def as_json(options = { })
  h = super(options)
  h[:finished]   = finished_items
  h[:unfinished] = unfinished_items
  h
end

if you wanted to use different names for the method-backed values.

If you care about XML and JSON, have a look at serializable_hash.

With Rails 4, you can do the following -

render json: @my_object.to_json(:methods => [:finished_items, :unfinished_items])

Hope this helps somebody who is on the later / latest version

Another way to do this is add this to your model:

def attributes
  super.merge({'unfinished' => unfinished_items, 'finished' => finished_items})
end

This would also automatically work for xml serialization. http://api.rubyonrails.org/classes/ActiveModel/Serialization.html Be aware though, you might want use strings for the keys, since the method can not deal with symbols when sorting the keys in rails 3. But it is not sorted in rails 4, so there shouldn't be a problem anymore.