How can I add attributes to rendered JSON from relationship without nesting? - ruby-on-rails

Question

Here's the problem: I'm using active record and returning some photo objects. The final consumer of these photo objects is going to be a mobile app.
The response needs to have thumbnail versions returned mobile developer has requested that the JSON coming back look like this..
{
"root_url":'http://place.s3.amazonaws.com/folder/',
"image_300":'image_300.jpg',
"image_600":'image_600.jpg',
"image_vga":'image_VGA.jpg',
"image_full":'image.jpg'
}
and not like this:
{
"root_url":'http://place.s3.amazonaws.com/folder/',
"thumbnails": {
"image_300":'image_300.jpg',
"image_600":'image_600.jpg',
"image_vga":'image_VGA.jpg',
"image_full":'image.jpg'
}
}
so far the easy way is to create columns for each of the thumbnails and wow it works. I don't like getting locked into that though because if we wanted different thumbnails later it would mean adding columns to the db etc. I would much prefer to either just specify the thumbnails in the model class OR have a separate table for thumbnails with one thumb per row of the table.
I've looked at delegate, composed_of, using GROUP_CONCAT in a join.., using :method=> in to_json .. none of these look like options. Is there an easy way to do this?
Basic model example:
class Photo < ActiveRecord::Base
has_many :thumbnails, :as => :thumbs_for #polymorphic
end
class Thumbnail < ActiveRecord::Base
# columns = name, filename
belongs_to :thumb_for, :polymorphic => true
end
So far the result looks like this based on the answer from jesse reiss
def as_json(options)
options ||= {} #even if you provide a default, it ends up as nil
hash = super(options.merge({:include => :thumbnails}))
if thumbs = hash.delete(:thumbnails)
thumbs.each {|t| hash.merge!({t['name']=>t['filename']})}
end
hash
end

Answer 1

You can customize the json serialization of an object pretty simply using the as_json method.
For this, you could do :
def as_json(*args)
hash = super(*args)
hash.merge!(hash.delete("thumbnails"))
end
Or you could do it super manually
def as_json(*args)
hash = super()
thumbnails.each do |thumb|
# build thumbnail json
end
end
You don't have to rely on ActiveRecord's super simplistic json serialization methods.