If I link some Student documents to a Class using a relationship field, then delete some Students, the relationship field on the Class will still contain their IDs. My feeling is that they should be cleaned up, but this behaviour might not be to everyone's taste, so it could be a field option on Types.Relationship.

It would also be nice to be warned before deleting a document if its ID is found in any relationship fields.

Source: https://github.com/keystonejs/keystone/issues/1152