Question

I’m running into an issue where I’m working with the as_json method, and how to efficiently return the object in JSON AND it’s belongs_to object as JSON as well, where the belongs_to object has its own belongs_to object. Code would probably explain it better.

The Not-Working Way

Alert class

class Alert < ActiveRecord::Base
    belongs_to :message
    # for json rendering
    def as_json(options={})
        super(:include => :message)
    end
end

Message class

 def as_json(options={})
    super( methods: [:timestamp, :num_photos, :first_photo_url, :tag_names],
           include: { camera: { only: [:id, :name] },
                      position: { only: [:id, :name, :address, :default_threat_level ]},
                      images: { only: [:id, :photo_url, :is_hidden]} })
  end

The problem with this first set up is that when I have an Alert object and call

alert.as_json()

I get all the attributes from Alert and all the attributes from Message, but none of the other attributes from Message that I want, like Camera, Position, etc.

Here’s the “It’s Working, But Probably Not Proper Design Way”

Alert Class

class Alert < ActiveRecord::Base

    belongs_to :message

    # for json rendering
    def as_json(options={})
        super().merge(:message => message.as_json)
    end
end

Messages Class

  # for json rendering
  def as_json(options={})
    super( methods: [:timestamp, :num_photos, :first_photo_url, :tag_names])
          .merge(:camera => camera.as_json)
          .merge(:position => position.as_json)
          .merge(:images => images.as_json)
  end

In this 2nd setup, I get all of Messages’s nested attributes like I want.

My question, am I missing some Rails Convention to do this properly? It seems like there would/should be an easier way.

Answer 1

Which version of Rails are you using? This is a known bug in older versions of Rails, supposedly fixed with this pull request. Your syntax looks right to me, so perhaps this is your problem?

As an aside, you may also want to checkout the new active_model_serializers from Jose Valim (Rails core member). It may at least enable you to work around your issue in a more elegant manner.