Relationships on Create for non ActiveRecord models

I guess more of a question.

I have this legacy app we are trying to slowly migrate to use jsonapi-resources. We have some bloated models we are splitting into smaller resources but also some models that point to Redis-backed models.

class RedisResource < JSONAPI::Resource
  attribute :attr_1
  attribute :attr_2

  # Override required. The underlying model is not based on
  # ActiveRecord.
  def self.find_by_key(key, options = {})
    context = options[:context]
    model = RedisModel.find(key)
    raise(JSONAPI::Exceptions::RecordNotFound, key) if model.nil?
    new(model, context)
  end
end

The problem though, is that somehow the relationship data is getting lost before reaching the Resource methods like: #replace_field(field_data).

All that info is available when I use an ActiveRecord model that uses the #belongs_to helper. But unsure what do I need to do/implement in my models to get the relationship assignment working.

Start with RedisResource#find_by_key and trace how relationship data reaches #replace_field(field_data) for non-ActiveRecord resources. Compare that path with an ActiveRecord resource using belongs_to; done when the issue’s relationship assignment works for the Redis-backed model, with the relevant behavior verified by tests.