Optimising Eager Load

Question

I have a Gallery that has_many MediaItems, each of which has_one Video or Photo.

I'm currently using the following query:

MediaItem.includes(gallery: {media_items: [:photo, :video]}).featured.rank(:position_in_featured)

However given that the media item will only ever have a Video or a Photo, I am always left with an unused eager load for whichever it doesn't have.

Is there a more efficient way of querying to avoid this unneeded include?

Note: I don't need a solution that involves Polymorphic associations. In this case they introduce too much complexity to other queries and don't fit the data model.

Answer 1

The best way to do this would probably be changing your data model to a polymorphic relationship and using a belongs_to association in MediaItem

class MediaItem < ActiveRecord::Base
  belongs_to :media_content, :polymorphic => true
end

class Photo < ActiveRecord::Base
  has_many :media_items, :as => :media_content
end

class Video < ActiveRecord::Base
  has_many :media_items, :as => :media_content
end

MediaItem.includes(gallery: {media_items: [:media_content]}).featured.rank(:position_in_featured)

For this to work you need to have a type media_content_id and media_content_type columns on MediaItem.

EDIT: There is even a note in the docs that eager loading should work in your case in the last paragraph of this section: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#module-ActiveRecord::Associations::ClassMethods-label-Eager+loading+of+associations