class Job < ActiveRecord::Base
  has_many :line_items
  ...
end
class DisposalJob < Job
  ...
end
class SupplyJob < Job
  ...
end
class LineItem < ActiveRecord::Base
  belongs_to :job, polymorphic: true
  ...
end

App.Job = DS.Model.extend
  line_items: DS.hasMany "line_item"
  ...

App.DisposalJob = App.Job.extend
  ...

App.SupplyJob = App.Job.extend
  ...

App.LineItem = DS.Model.extend
  job: DS.belongsTo "job",
    polymorphic: true

class BaseSerializer < ActiveModel::Serializer
  ActiveModel::Serializer.setup do |config|
    config.embed = :ids
    config.embed_in_root = true
  end
end

class JobSerializer < BaseSerializer
  ...
  has_many :line_items
end

class DisposalJobSerializer < JobSerializer
  ...
end

class SupplyJobSerializer < JobSerializer
  ...
end

class LineItemSerializer < BaseSerializer
  belongs_to :job, polymorphic: true
end