title: How do I test a model with a polymorphic belongs_to association in isolation?
body: <p>Up front, I'm using:</p>
  
  <ul>
  <li>Ruby 2.3.1</li>
  <li>Rails 5</li>
  <li>rspec-rails 3.5</li>
  </ul>
  
  <p>I've set up a mountable Rails engine with a polymorphic model that I want to use in other engines. The model class looks like this:</p>
  
  
  
  <pre class="lang-ruby prettyprint-override"><code>module Geo
    class Location &lt; ApplicationRecord
      belongs_to :locatable, polymorphic: true
    end
  end
  </code></pre>
  
  <p>In my specs I'd like to make sure that I have a valid model, however, within the engine I have no other model that is associated with <code>Geo::Location</code>.</p>
  
  <p>How can I set up a dummy class for testing validity (<code>belongs_to</code> require presence) or what are good testing strategies that you have used?</p>