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Luisepvs
Asked: 2020-12-31 23:28:24 +0800 CST 2020-12-31 23:28:24 +0800 CST

Query on relation in Ruby on Rails

  • 772

I'm just starting out on Rails and haven't been able to find a relationship.

I have 3 tables: Video, donations and users. A user can put X videos and each video can have several donations.

  • In video I haveuser_id
  • In Donations I have the user_idandvideo_id

My models are:

class Donation < ActiveRecord::Base
  belongs_to :user
  belongs_to :video
end

class Video < ActiveRecord::Base
  belongs_to :user
  has_many :donation
end

class User < ActiveRecord::Base 
  has_many :video
  has_many :donation
end

I can get the user of each video without problems:

video.user.name

The problem occurs when I want to get the title of the video of a donation:

 <% @donaciones.each do |d| %>
      <%= d.video.titulo %><br>
 <% end %>

always gives me:

undefined method `titulo' for nil:NilClass

The title is in the table videos, I only have 1 record.

Is there a way to be able to see the full query being performed on the error page?

ruby-on-rails

1 Answers

  1. Best Answer

    What the code donacion.video.titulodoes is look at the value video_idof the donation attribute and search the videos table for the corresponding video.

    The problem is that in this case the video does not exist (because it was deleted for example) or the donation does not have an associated video, in this case donacion.videoit is nil and you get that error.

    To avoid this, you can make the required relationship (to avoid having donations without videos in the database):

    class Donation < ActiveRecord::Base
  belongs_to :video, required: true
end

    You should also see how to prevent videos from being deleted and leaving orphan donations, for that you would have to use the :dependenthas_many option, depending on how you want to solve the problem.

    class Videos < ActiveRecord::Base
   has_many :donations, dependent: :restrict_with_exception
end

    Lastly, if you don't mind that there is no associated video, you can use a null object (from the "null-pattern" pattern) by overriding the method video:

    class Donation < ActiveRecord::Base
  def video
    super || NullVideo
  end
end
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