gistfile1.rb

class Festival < ActiveRecord::Base
  attr_accessible :name, :client_id

  belongs_to :client

  has_many :covers
  has_many :photos

  def self.with_covers
    where("covers_count > 0")   # or some other SQL with a COUNT and group maybe that filters correctly
  end

  def as_json(opts = {})
    if !covers.empty?
      opts = opts.merge({
        :except => [:created_at, :updated_at],
        :include => { 
          :covers => {
            :methods => [:image_url, :thumbnail_url],
            :except => [:created_at, #:updated_at, 
              :main_text_block_id, :left_text_bar_id,
              :first_left_text_block_id, :second_left_text_block_id, :third_left_text_block_id, 
              :first_right_text_block_id]
          }
        }
      })
      super(opts)
    end
  end
end

render json: @festivals.with_covers