title: Django Model set foreign key to a field of another Model
body: <p>Is there any way to set a foreign key in django to a <em>field</em> of another model?</p>
  
  <p>For example, imagine I have a ValidationRule object.  And I want the rule to define what field in another model is to be validated (as well as some other information, such as whether it can be null, a data-type, range, etc.)</p>
  
  <p>Is there a way to store this field-level mapping in django?</p>