Question

I have a post model that has a virtual attribute that I would like to set and then include in a response to a JSON call to my post#index action. I can’t seem to get the virtual attribute to be included in the response.

class Post < ActiveRecord::Base
  attr_accessible :height
  attr_accessor :m_height
end

class PostsController < ApplicationController
  respond_to :html, :json, :js

  def index
    story = Story.find(params[:story_id])
    @posts = story.posts.where("posts.id >= ?", 100)
    @posts.each do |post|
      post.m_width = post.height * 200
    end
    results = { :total_views => story.total_views,  
                :new_posts => @posts }
    respond_with(results)
  end
end

I think that I must need something similar to @post.to_json(:methods => %w(m_width)), but I don’t see how to use :methods in a respond_with

Answer 1

This seems to provide the answer. Implement a to_json and to_xml in your models, as appropriate, with definitions like:

There’s a better answer implied here.

Following code stolen from the post:

  def as_json(options={})
    super(options.merge(:methods => [...], :only => [...], :include => [...])
  end

to_json won’t be called on your model in this case, from what I can tell in the source, but as_json will be, in the process of serialization.

So, here’s what happens, in overview form:

1. You call respond_with with the results hash you’ve constructed.
2. Rails (ActionController) calls to_json on that.
3. to_json sends you over to JSON::Encoding which keeps calling as_json all the way down until everything is JSONified.

That’s why there was the confusion about to_json and as_json in an earlier version of this answer.