added form validation function

disable_field_autocomplete.module

@@ -6,19 +6,87 @@
  */
 
 use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\taxonomy\Entity\Term;
 
 /**
  * Implements hook_help().
  */
-function doi_field_formatter_help($route_name, RouteMatchInterface $route_match) {
+function disable_field_autocomplete_help($route_name, RouteMatchInterface $route_match) {
   switch ($route_name) {
-    // Main module help for the facet_field_formatter module.
     case 'help.page.facet_field_formatter':
       $output = '';
       $output .= '<h3>' . t('About') . '</h3>';
-      $output .= '<p>' . t('Format string and text fields as doi links') . '</p>';
+      $output .= '<p>' . t('Disable autocomplete for TypedRelation fields') . '</p>';
       return $output;
 
     default:
   }
 }
+
+/**
+ * Implements hook_form_alter().
+ */
+function disable_field_autocomplete_form_node_islandora_object_edit_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
+  $form['#validate'][] = 'disable_field_autocomplete_node_edit_validate';
+}
+
+
+/**
+ * Extra Islandora Ojbect node validation required due to lacking autocomplete.
+ * @param $form
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ * @return void
+ */
+function disable_field_autocomplete_node_edit_validate(&$form, \Drupal\Core\Form\FormStateInterface $form_state){
+  $contributors = &$form_state->getValue('field_linked_agent');
+  $contributor_count = 0;
+  foreach ($contributors as &$contributor) {
+    if (is_array($contributor) && isset($contributor['target_id']) && is_array($contributor['target_id'])) {
+      $entity = ($contributor['target_id']['entity']);
+      $name = $entity->getName();
+      if (isset($name)) {
+        try {
+          $term_id = disable_field_autocomplete_get_term_id($name);
+          if (isset($term_id) && $term_id > 0) {
+            $contributor['target_id'] = $term_id;
+          }
+        } catch (Exception $e) {
+          $form_state->setError($form['field_linked_agent']['widget'][$contributor_count],
+            t('The entered Term %name is not unique across Vocabularies.  You will need to include
+      the term id in brackets.  For Example "%name (119)", where 119 is the term id.', array('%name' => $name)));
+        }
+      }
+    }
+    $contributor_count++;
+  }
+  return;
+}
+
+/**
+ * Checks for existing terms.
+ *   Since autocommplete is off we need to check ourselves.
+ * @param $term_name
+ * @return int
+ * @throws \Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException
+ * @throws \Drupal\Component\Plugin\Exception\PluginNotFoundException
+ */
+function disable_field_autocomplete_get_term_id($term_name) {
+  $storage = \Drupal::entityTypeManager()->getStorage('taxonomy_term');
+  $terms = $storage->loadByProperties([
+    'name' => $term_name,
+    'vid' => ['Person', 'Contributors', 'Host Contributors', 'Series Contributors'],
+  ]);
+
+  if ($terms) {
+    if (count ($terms) > 1) {
+      throw new Exception ('The entered Term is not unique across Vocabularies.  You will need to include
+      the term id in brackets.  For Example "term value (119)", where 119 is the term id.');
+    }
+    $term = reset($terms);
+  }
+  if(!isset($term)) {
+    return -1;
+  }
+  $tid = $term->id();
+  return $tid;
+}

src/Plugin/Field/FieldWidget/DisableAutocompleteWidget.php

@@ -5,6 +5,7 @@ namespace Drupal\disable_field_autocomplete\Plugin\Field\FieldWidget;
 use Drupal\controlled_access_terms\Plugin\Field\FieldWidget\TypedRelationWidget;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\taxonomy\Entity\Term;
 
 /**
  * Plugin implementation of the 'chosen_select' widget.
@@ -24,9 +25,8 @@ class DisableAutocompleteWidget extends TypedRelationWidget {
    */
   public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
     $widget = parent::formElement($items, $delta, $element, $form, $form_state);
+    // disable autocomplete
     $widget['target_id']['#selection_settings']['target_bundles'] = [];
-
     return $widget;
   }
-
 }