Prevent content with a content reference from referencing itself

Entity reference for Drupal 7 has a ready-to-be-committed patch for this: https://www.drupal.org/project/entityreference/issues/1364802

And a “Needs work” patch for Drupal core:

Add setting to EntityReferenceSelection plugins to prevent references to referencing entity https://www.drupal.org/project/drupal/issues/2280479

Ah but there’s a contrib fix: https://www.drupal.org/project/entity_reference_validators

Needed for Find It: A service provider or content editor is prevented from creating a circular reference between organizations.

as part of A content editor can associate an organization with a parent organization.