Question

I am using Mongoid with Rails 3. What would be the best way to limit the number of embedded objects (photos) that can be stored within each parent object (album)?

class Album
  include Mongoid::Document
  embeds_many :photos
end

class Photo
  include Mongoid::Document
  embedded_in :album, :inverse_of => :photos
end

With ActiveRecord, I would do something like:

has_many :photos, :before_add => :enforce_photo_limit

private

def enforce_photo_limit
  raise "Too many photos" if self.photos.count >= 50
end

…but this isn’t supported by Mongoid.

Any suggestions much appreciated.

Thanks.

Answer 1

Mongoid includes ActiveModel::Validations, so you should be able to use the methods contained in that module:

class Album
  include Mongoid::Document
  embeds_many :photos

  validate :less_than_fifty_photos

  def less_than_fifty_photos
    errors.add(:base, "Too many photos") if self.photos.count >= 50
  end
end

More info: http://mongoid.org/docs/validation.html