Add virtual attribute to json output

Let's say I have an app that handles a TODO list. The list has finished and unfinished items. Now I want to add two virtual attributes to the list object; the count of finished and unfinished items in the list. I also need these to be displayed in the json output.

I have two methods in my model which fetches the unfinished/finished items:

def unfinished_items 
  self.items.where("status = ?", false) 
end 

def finished_items 
  self.items.where("status = ?", true) 
end

So, how can I get the count of these two methods in my json output?

I'm using Rails 3.1

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.

just close all of your data into one hash, like

render json: {items: items, finished: finished, unfinished: unfinished}

I just thought I'd provide this answer for anyone like myself, who was trying to integrate this into an existing as_json block:

  def as_json(options={})
    super(:only => [:id, :longitude, :latitude],
          :include => {
            :users => {:only => [:id]}
          }
    ).merge({:premium => premium?})

Just tack .merge({}) on to the end of your super()

This will do, without having to do some ugly overridings. If you got a model List for example, you can put this in your controller:

  render json: list.attributes.merge({
                                       finished_items: list.finished_items,
                                       unfinished_items: list.unfinished_items
                                     })

As Aswin listed above, :methods will enable you to return a specific model's method/function as a json attribute, in case you have complex assosiations this will do the trick since it will add functions to the existing model/assossiations :D it will work like a charm if you dont want to redefine as_json

Check this code, and please notice how i'm using :methods as well as :include [N+Query is not even an option ;)]

render json: @YOUR_MODEL.to_json(:methods => [:method_1, :method_2], :include => [:company, :surveys, :customer => {:include => [:user]}])

Overwritting as_json function will be way harder in this scenario (specially because you have to add the :include assossiations manually :/

def as_json(options = { })
end

If you want to render an array of objects with their virtual attributes, you can use

render json: many_users.as_json(methods: [:first_name, :last_name])

where first_name and last_name are virtual attributes defined on your model