title: How to let a Reference Field accept multiple Document schemas in MongoEngine?
body: <p>Context: I am writing an API (using Flask and MongoEngine) with multiple account types, including perhaps buildings. I need the database to hold some temporary accounts until a particular building registers.</p>
  
  <p>This is how I've been referencing just one type of user:</p>
  
  <pre><code>current_holder_of_stuff = ReferenceField(ActiveUser)
  </code></pre>
  
  <p>I know GenericReferenceField is also an option, but what if I only want to allow two types of references? Is there anything like:</p>
  
  <pre><code>current_holder_of_stuff = ReferenceField(ActiveUser, TempUser)
  </code></pre>
  
  <p>Muchos thankos! </p>