How do I find "where-used" / many-to-one related entities?

Question

A field in my Content Type A references a Content Type B.

I'd like to find, in each instance of Content Type B, a list of all the As that reference it at a particular point in time.

[Update]: I made the question less ambiguous in the light of the answer I found with help from others here.

Answer 1

If you want a quick solution in code, you can use the same way as in drupal 7:

Add a field (configurable in view modes):

hook_entity_extra_field_info

Put the code for the field in here:

hook_ENTITY_TYPE_view

Use this code:

How to check if an article is referenced from a collection?

Answer 2

In D7, Entity Views Attachment was the most popular solution for this. It has no D8 port yet however.

D8 modules which offer this functionality include:

• Display Suite. It enables you to add a view as a dynamic field; that view could use a contextual filter to focus its content on the current node URL.

• Viewfield, as you suggested. You use a token to specify a default view, and choose to "force the default" to prevent content editors from changing the selected view.

• Display fields, an interesting new D8 module. It allows you to place views as fields on a node, and is meant to allow contextual arguments, but I've not been able to set this up properly (see the issue)

Answer 3

This code is in my agenda.module. The where-used relation is used as follows in the form validation handler that is called when the user clicks the Delete button on the delete confirmation form:

The Agenda Node cannot be deleted if there are any Meeting Nodes that refer to it.

use Drupal\Core\Form\FormStateInterface;

function agenda_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  switch ($form_id) {
    // ...
    case "node_agenda_delete_form":
      $form['#validate'][] = '_agenda_delete_validation';
      break;
    // ...
  }
}

function _agenda_delete_validation(&$form, FormStateInterface $form_state) {
  $agenda_node = $form_state->getFormObject()->getEntity();
  $agenda_name = $agenda_node->getTitle();
  $agenda_node_id = $agenda_node->id();
  $query = \Drupal::entityQuery('node')
    ->condition('type', 'meeting')
    ->condition('field_agenda', $agenda_node_id);
  $nids = $query->execute();
  $count_nids = count($nids);
  if ($count_nids > 0) {
    $msg_stem = ($count_nids == 1) ? 'There is one Meeting that references "' : 'There are '.$count_nids.' Meetings that reference "';
    $form_state->setError($form, $msg_stem.$agenda_name.'". It cannot be deleted yet.' );
  }
}