Auto-populate material type term reference from a material inventory asset.

modules/quantity/material/farm_quantity_material.services.yml

@@ -0,0 +1,5 @@
+services:
+  farm_quantity_material.quantity_subscriber:
+    class: Drupal\farm_quantity_material\EventSubscriber\QuantityEventSubscriber
+    tags:
+      - { name: 'event_subscriber' }

modules/quantity/material/src/EventSubscriber/QuantityEventSubscriber.php

@@ -0,0 +1,62 @@
+<?php
+
+namespace Drupal\farm_quantity_material\EventSubscriber;
+
+use Drupal\quantity\Event\QuantityEvent;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Perform actions when quantity entities are saved/deleted.
+ */
+class QuantityEventSubscriber implements EventSubscriberInterface {
+
+  /**
+   * {@inheritdoc}
+   *
+   * @return array
+   *   The event names to listen for, and the methods that should be executed.
+   */
+  public static function getSubscribedEvents() {
+    return [
+      QuantityEvent::PRESAVE => 'quantityPresave',
+    ];
+  }
+
+  /**
+   * Copy material_type from a material inventory asset to material quantity.
+   *
+   * @param \Drupal\quantity\Event\QuantityEvent $event
+   *   Quantity event.
+   */
+  public function quantityPresave(QuantityEvent $event) {
+
+    $quantity = $event->quantity;
+
+    // Bail if not a material quantity.
+    if ($quantity->bundle() !== 'material') {
+      return;
+    }
+
+    // Bail if there is no inventory field or if it is empty.
+    if (!$quantity->hasField('inventory_asset') || $quantity->get('inventory_asset')->isEmpty()) {
+      return;
+    }
+
+    // Get the referenced inventory asset.
+    /** @var \Drupal\asset\Entity\AssetInterface[] $assets */
+    $assets = $event->quantity->get('inventory_asset')->referencedEntities();
+    $asset = reset($assets);
+
+    // Bail if not a material asset.
+    if (empty($asset) || $asset->bundle() !== 'material') {
+      return;
+    }
+
+    // Copy the material asset material_type field to the material quantity.
+    if (!$asset->get('material_type')->isEmpty()) {
+      $material_type = $asset->get('material_type')->getValue();
+      $quantity->set('material_type', $material_type);
+    }
+  }
+
+}