Entity Reference Referential Integrity

This module solves the problem of deleting parent entities (nodes, taxonomy terms, users, etc.) that are referred to by other entities through taxonomy term reference or entity reference fields (see https://www.drupal.org/project/entityreference/issues/1368386, for example). Drupal will not prevent a user from deleting such 'parent entities', but this module will, provided some kind of delete confirmation form is used. It hooks into various entity delete confirmation forms, and does a check of the entity or entities about to be deleted as to whether or not they are referred to by any other entities, and if so, shows the referrers (the child entities) to the user if the current user has view permissions on the referring entities and fields, or if not, just that there is referring content, and either

1. disables the delete button on the form if any of the referring fields are required, or where the current user does not have update permissions on either the referring entities or referring fields, or
2. where the referring fields are all optional and the current user has permissions to update both the referring entities and the referring field on those entities, changes the text of the delete button to indicate all referring fields will be set to empty.

Required fields are noted with a red asterisk in the list of referrers, and in the case where a user has insufficient permissions to update non-required fields, the user is informed. Previous revisions are also checked.

Note that this module currently implements 'Restrict' referential integrity where any of the referring fields' settings are set as 'Required', and 'Set Null' if all of the referring fields' settings are set as not required. 'Cascade' is not (yet) implemented.

API

The module already handles node, taxonomy term, and user delete forms, both single and multi-versions (including Taxonomy Manager's delete), and has an API hook hook_erri_info() for other modules that use the Drupal confirm_form() function to create their delete forms, or at least have elements either identical to or like the ['description']['#markup'] form element and the ['actions']['submit'] button used for the delete (see https://api.drupal.org/api/drupal/modules%21system%21system.module/funct...). If your module's custom delete confirmation form doesn't have those elements identical to those supplied by confirm_form(), you can specify equivalent elements in hook_erri_info(). Note that the description element must be one that permits the concatenation of text, and the submit element must be of #type 'submit'.

A module can implement hook_erri_reference_fields() to specify additional reference fields (see erri.api.php) so that this module will enforce referential integrity for them as well (where they are referencing either an entity or a taxonomy term). For example, if your module implements a compound field structure where one of the components is an entity-reference component, hook_erri_reference_fields() will allow your module to leverage this module's integrity checking. Such fields must be created through the field API, as this module relies on other aspects of the Field API to do its work.

A module can call the function erri_get_child_entities, passing it the specified parameters, to get a two-level array of child entities for a particular parent, keyed on the field referencing the parent, and then act accordingly. For example:

$fields = erri_get_child_entities('node', $parent->nid, TRUE, 'customer');
if ( !empty($fields) ) {
  foreach ( $fields as $field_name => $children ) {
    // $children is an array with each element an array of entity_type, bundle, entity_id, etc. that all point to node $parent through field $field_name.
    ...
  }
}

Installation

Install this module like any other, and activate it. There's nothing else to do. The module will begin working immediately.

Version

This is a dev version right now, as I'd like to have some folks review it before I make it a production version. That said, I'm using it on a couple of sites, and so far it's doing the job. Please review and comment!