Resource interacts with HasOne and HasMany differently

I have a model (User) that I want to expose on an API to a certain class of users. Those users will always have a given permission and expiration, so I want to set those on the server, rather than expose uneditable fields to the client.

class UserResource < JSONAPI::Resource
  has_many :permissions
  has_one  :expiration, foreign_key_on: :related

  before_create :add_defaults

  def add_defaults
    self.permissions << Permission.find_by(title: 'Admin')
    self.model.expiration = Expiration.new(date: Date.new(2015,12,30))
  end

I think it's odd that I need to call the underlying model in order to interact with has_one objects, and I suspect this is the case for created objects, but I haven't gotten that far yet. Ideally, I could call self.expiration= and I would get behavior similar to self.permissions<<.

Am I just using JSONAPI::Resource incorrectly, or is this something worth pursuing?

Start with the UserResource relationship declarations and its before_create callback, comparing the has_many permissions interaction with the has_one expiration interaction. Determine the expected resource-level assignment and creation behavior, then verify that has_one handling matches the requested has_many-like behavior with focused tests.