Rails 4: Order has_many in serializer

Question

I have a serializer with a has_many association and I want to order the dependent model. For some reason I get an undefined method key?' error:

class API::ClientSerializer < ActiveModel::Serializer
    has_many :check_ins, -> { order(:week) }
end

How else do I order the check_ins by week?

Answer 1

class API::ClientSerializer < ActiveModel::Serializer
  has_many :check_ins do
    object.check_ins.order(:week)
  end 
end