How to modify as_indexed_json values to run Ruby code

I am trying to import my data using the elasticsearch-rails and elasticsearch-model gems.

Here's what I have in my question.rb:

belongs_to :user

...

def as_indexed_json(options={})
  as_json(
    user_name: user.name,
    user_email: user.email
  )
end

When I run the import Question.import, I get this error:

NoMethodError: undefined method `name' for nil:NilClass

How do I import my data in a way that allows me to run some ruby against the ActiveRecord object? There are other nested associations that are 2 or 3 degrees away, and I would like to denormalize that data in Elasticsearch.

I can't use a transform on the import because that will only work at import time. I need the code to run on create and update events too.