How to customize to_json response in Rails 3

Question

I am using respond_with and everything is hooked up right to get data correctly. I want to customize the returned json, xml and foobar formats in a DRY way, but I cannot figure out how to do so using the limited :only and :include. These are great when the data is simple, but with complex finds, they fall short of what I want.

Lets say I have a post which has_many images

def show
  @post = Post.find params[:id]
  respond_with(@post)
end

I want to include the images with the response so I could do this:

def show
  @post = Post.find params[:id]
  respond_with(@post, :include => :images)
end

but I dont really want to send the entire image object along, just the url. In addition to this, I really want to be able to do something like this as well (pseudocode):

def show
  @post = Post.find params[:id]
  respond_with(@post, :include => { :foo => @posts.each.really_cool_method } )
end

def index
  @post = Post.find params[:id]
  respond_with(@post, :include => { :foo => @post.really_cool_method } )
end

… but all in a DRY way. In older rails projects, I have used XML builders to customize the output, but replicating it across json, xml, html whatever doesnt seem right. I have to imagine that the rails gurus put something in Rails 3 that I am not realizing for this type of behavior. Ideas?

Answer 1

You can override as_json in your model. Something like:

class Post < ActiveRecord::Base
  def as_json(options = {})
    {
      attribute: self.attribute, # and so on for all you want to include
      images:    self.images,    # then do the same `as_json` method for Image
      foo:       self.really_cool_method
    }
  end
end

And Rails takes care of the rest when using respond_with. not entirely sure what options gets set to, but probably the options you give to respond_with (:include, :only and so on)

Answer 2

Probably too late, but I found a more DRY solution digging through the rails docs. This works in my brief tests, but may need some tweaking:

# This method overrides the default by forcing an :only option to be limited to entries in our
# PUBLIC_FIELDS list
def serializable_hash(options = nil)
  options ||= {}
  options[:only] ||= []
  options[:only] += PUBLIC_FIELDS
  options[:only].uniq!
  super(options)
end

This basically allows you to have a list of fields that are allowed for your public API, and you cannot accidentally expose the whole object. You can still expose specific fields manually, but by default your object is secure for .to_json, .to_xml, etc.