Mandatory field (A or B) in a content type I have a content type (external_document) that has an entity relation to an "event" and an entity relation to a "project."

I would like to be able to have some validation rule saying: All the documents must be "linked" to either an event or a project.

I am unclear on the following:


How  to do it on D7?
What is the proper name for that in Drupal so I can search it?
Is it the "Drupal" way of doing the content type with 2 entity relations?
You could use the Field validation module to take out some of the legwork.  


  This module adds an extra tab to each field instance, allowing you to
  specify validation rules for your field instances.


There's a validation rule built in for 


  Require at least one of several fields


which is exactly what you're after.