title: How can I create a model in django that has a foreign key to one of multiple models?
body: <p>I want to create a foreign key that can have a reference to exactly one of three other models. example:</p>
  
  <pre><code>user
    fav_animal: foreign_key(bear, cat, dog)
  
  bear
    name
  
  cat
   name
  
  dog
    name
  </code></pre>
  
  <p>How can I achieve this in django? Do I just create 3 nullable foreign keys? Then how do I enforce that always exactly one of them is set?</p>