album model

album.rb

class Album < ActiveRecord::Base
  has_many   :assets
  belongs_to :user

  def self.formatted
    all.map do |album|
      {:id => album.id,
       :name => album.name}
    end
  end
  
  def formatted_assets(favorite_ids)
    assets.map {|asset| asset.to_formatted(favorite_ids)}
  end
  
end

asset.rb

class Asset < ActiveRecord::Base
  belongs_to :user
  has_many :favorites
  
  named_scope :photos, {:conditions => {:type => "Photo"}}
  named_scope :videos, {:conditions => {:type => "Video"}}
  named_scope :audios, {:conditions => {:type => "Audio"}}
  
  def to_formatted(favorite_ids = [])
    {:id => self.id, 
     :name => self.name,
     :album_id => self.album_id, 
     :src => self.attachment.url(:small),
     :is_private => self.is_private,
     :is_favorite => favorite_ids.include?(self.id) }
  end
  
end

photos_controller.rb

class PhotosController < ApplicationController
  
  def index
    @favorite_ids = current_user.favorites.map(&:asset_id)
    
    @photo_album = Album.find(params[:photo_album_id])
    @photos = @photo_album.formatted_assets(@favorite_ids)

    render :json => @photos.to_json
  end
  
  
end