To understand what delegate work, let’s see the following code:

  %td
    = link_to queue_item.video_title, queue_item.video
  %td
    = link_to queue_item.category_name, queue_item.category

Suppose we had some template in the view that we call from our queue_item model. We want the video_title from it. Actually, we can just call queue_item.video.title or queue_item.category.name. But we can make them more friendly by use delegate in the model.

  class QueueItem < ActiveRecord::Base
    belongs_to :user
    belongs_to :video

    delegate :category, to: :video
    delegate :title, to: :video, prefix: :video
    #queue_item.video_name
   #...
  end


  class Video < ActiveRecord::Base
    belongs_to :category
    has_many :reviews, -> {order("created_at DESC")}
    validates :title, presence: true
    validates :description, presence: true
  #...
  end

If you read this line: delegate :title, to: :video, prefix: :video, it tells you that you can find the title method in the video model, and in this model, you add the prefix video_title, so queue_item.video_title now is calling the video.title

take away

• Delegation is particularly useful with Active Record associations.
• Multiple delegates to the same target are allowed