Model Methods in Fixtures - Rails 4.2 to 5

In the process of migrating an app from Rails 4.2 to 5. Getting this when trying to run tests:

NameError: uninitialized constant ActiveRecord::FixtureSet::RenderContext::User

test/fixtures/users.yml:6:in `get_binding'

users.yml:

one:
  name: "Tim Smith"
  email: tim@smith.com
  password_digest: <%= User.digest('password') %>
  id: 1

It's failing on the User.digest call, which is a method in the User model. This worked in 4.2, but in 5 it can't find that method anymore. Any advice? Is there something more I need to declare in test_helper.rb? Here is what's there now:

ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
require 'rails/test_help'
require 'application_helper'

class ActiveSupport::TestCase
  # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
  fixtures :all
  include ApplicationHelper