title: How to add Check Constraints for Django Model fields?
body: <p>While subclassing <code>db.models.Model</code>, sometimes it's essential to add extra checks/constraints.</p>
  <p>For example, I have an <code>Event</code> model with <code>start_date</code> and <code>end_date</code>: I want to add validation into the fields or the model so that <code>end_date &gt; start_date</code>.</p>
  <p>At least I know this can be done outside the <code>models.Model</code> inside the <code>ModelForm</code> validation. But how to attach to the fields and the <code>models.Model</code>?</p>