Active Model Serializers belongs_to

Question

This question pertains to AMS 0.8

I've got two models:

class Subject < ActiveRecord::Base
  has_many :user_combinations
  has_ancestry
end

class UserCombination < ActiveRecord::Base
  belongs_to :stage
  belongs_to :subject
  belongs_to :user
end

And two serializers:

class UserCombinationSerializer < ActiveModel::Serializer
      attributes :id
      belongs_to :stage
      belongs_to :subject
end

class SubjectSerializer < ActiveModel::Serializer
  attributes :id, :name, :description, :subjects

  def include_subjects?
    object.is_root?
  end

  def subjects
    object.subtree
  end
end

When a UserCombination is serialized, I want to embed the whole subtree of subjects.

When I try to use this setup I get this error:

undefined method `belongs_to' for UserCombinationSerializer:Class

I tried changing the UserCombinationSerializer to this:

class UserCombinationSerializer < ActiveModel::Serializer
  attributes :id, :subject, :stage
end

In this case I get no errors, but the subject is serialized in the wrong way - not using the SubjectSerializer.

My questions:

1. Shouldn't I be able to use a belongs_to relation in the serializer?
2. If not - how can I get the wanted behaviour - embedding the subject tree using the SubjectSerializer?

Answer 1

This is not really elegant but it seems to be working :

class UserCombinationSerializer < ActiveModel::Serializer
  attributes :id, :stage_id, :subject_id

  has_one :subject
end

I don't really like calling has_one whereas it's actually a belongs_to association :/

EDIT: Disregard my comment about has_one/belongs_to ambiguity, the doc is actually pretty clear about it: http://www.rubydoc.info/github/rails-api/active_model_serializers/frames

Answer 2

In Active Model Serializer 0-10-stable, belongs_to is now available.

belongs_to :author, serializer: AuthorPreviewSerializer
belongs_to :author, key: :writer
belongs_to :post
belongs_to :blog
def blog
  Blog.new(id: 999, name: 'Custom blog')
end

https://github.com/rails-api/active_model_serializers/blob/0-10-stable/docs/general/serializers.md#belongs_to

So you could do:

class UserCombinationSerializer < ActiveModel::Serializer
  attributes :id
  belongs_to :stage, serializer: StageSerializer
  belongs_to :subject, serializer: SubjectSerializer
end

Answer 3

What if you try with something like this:

class UserCombinationSerializer < ActiveModel::Serializer
  attributes :subject,
             :stage,
             :id

  def subject
    SubjectSerializer.new(object.subject, { root: false } )
  end

  def stage
    StageSerializer.new(object.stage, { root: false } )
  end
end