Validate that only one birth log can reference an asset.

modules/log/birth/farm_birth.module

@@ -7,6 +7,7 @@
 
 use Drupal\Core\Entity\Display\EntityViewDisplayInterface;
 use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\Core\Form\FormStateInterface;
 
 /**
@@ -38,3 +39,15 @@ function farm_birth_log_view_alter(array &$build, EntityInterface $entity, Entit
     $build['asset']['#title'] = t('Children');
   }
 }
+
+/**
+ * Implements hook_entity_base_field_info_alter().
+ */
+function farm_birth_entity_base_field_info_alter(&$fields, EntityTypeInterface $entity_type) {
+  /** @var \Drupal\field\Entity\FieldConfig[] $fields */
+
+  // Validate that only one birth log references an asset.
+  if ($entity_type->id() == 'log' && !empty($fields['asset'])) {
+    $fields['asset']->addConstraint('UniqueBirthLog');
+  }
+}

modules/log/birth/src/Plugin/Validation/Constraint/UniqueBirthLogConstraint.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Drupal\farm_birth\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+
+/**
+ * Checks that only one birth log references an asset.
+ *
+ * @Constraint(
+ *   id = "UniqueBirthLog",
+ *   label = @Translation("Unique birth log", context = "Validation"),
+ * )
+ */
+class UniqueBirthLogConstraint extends Constraint {
+
+  /**
+   * The default violation message.
+   *
+   * @var string
+   */
+  public $message = '%child already has a birth log. More than one birth log cannot reference the same child.';
+
+}

modules/log/birth/src/Plugin/Validation/Constraint/UniqueBirthLogConstraintValidator.php

@@ -0,0 +1,73 @@
+<?php
+
+namespace Drupal\farm_birth\Plugin\Validation\Constraint;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Validates the UniqueBirthLog constraint.
+ */
+class UniqueBirthLogConstraintValidator extends ConstraintValidator implements ContainerInjectionInterface {
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * Constructs a UniqueBirthLogConstraintValidator object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('entity_type.manager')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($value, Constraint $constraint) {
+    /** @var \Drupal\Core\Field\FieldItemListInterface[] $value */
+    /** @var \Drupal\farm_birth\Plugin\Validation\Constraint\UniqueBirthLogConstraint $constraint */
+    foreach ($value as $item) {
+
+      // Get the referenced asset ID.
+      $item_value = $item->getValue();
+      $asset_id = $item_value['target_id'] ?? FALSE;
+
+      // If there is no asset, skip.
+      if (empty($asset_id)) {
+        continue;
+      }
+
+      // Perform an entity field query to find logs that reference the asset.
+      $query = $this->entityTypeManager->getStorage('log')->getQuery();
+      $ids = $query->condition('type', 'birth')
+        ->condition('asset', $asset_id)
+        ->execute();
+
+      // If more than 1 birth logs reference the asset, add a violation.
+      if (count($ids) > 1) {
+        $asset = $this->entityTypeManager->getStorage('asset')->load($asset_id);
+        $this->context->addViolation($constraint->message, ['%child' => $asset->label()]);
+      }
+    }
+  }
+
+}

modules/log/birth/tests/src/Kernel/BirthTest.php

@@ -3,6 +3,7 @@
 namespace Drupal\Tests\farm_birth\Kernel;
 
 use Drupal\asset\Entity\Asset;
+use Drupal\Component\Render\FormattableMarkup;
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\log\Entity\Log;
 use Drupal\taxonomy\Entity\Term;
@@ -36,6 +37,7 @@ class BirthTest extends KernelTestBase {
     'user',
     'taxonomy',
     'text',
+    'views',
   ];
 
   /**
@@ -130,4 +132,54 @@ class BirthTest extends KernelTestBase {
     $this->assertEquals($timestamp, $child2->get('birthdate')->value);
   }
 
+  /**
+   * Test that only one birth log can reference an asset.
+   */
+  public function testUniqueBirthLogConstraint() {
+
+    // Create a Cow animal type term.
+    /** @var \Drupal\taxonomy\TermInterface $cow */
+    $cow = Term::create([
+      'name' => 'Cow',
+      'vid' => 'animal_type',
+    ]);
+    $cow->save();
+
+    // Create an asset.
+    /** @var \Drupal\asset\Entity\AssetInterface $animal */
+    $asset = Asset::create([
+      'name' => $this->randomMachineName(),
+      'type' => 'animal',
+      'animal_type' => ['tid' => $cow->id()],
+      'status' => 'active',
+    ]);
+    $asset->save();
+
+    // Create a birth log that references the asset.
+    $log1 = Log::create([
+      'type' => 'birth',
+      'timestamp' => \Drupal::time()->getRequestTime(),
+      'asset' => [['target_id' => $asset->id()]],
+    ]);
+    $log1->save();
+
+    // Confirm that there were no validation errors.
+    $errors = $log1->validate();
+    $this->assertCount(0, $errors);
+
+    // Create a second birth log that references the asset.
+    $log2 = Log::create([
+      'type' => 'birth',
+      'timestamp' => \Drupal::time()->getRequestTime(),
+      'asset' => [['target_id' => $asset->id()]],
+    ]);
+    $log2->save();
+
+    // Confirm that validation fails.
+    $errors = $log2->validate();
+    $this->assertCount(1, $errors);
+    $this->assertEquals(new FormattableMarkup('%child already has a birth log. More than one birth log cannot reference the same child.', ['%child' => $asset->label()]), $errors[0]->getMessage());
+    $this->assertEquals('asset', $errors[0]->getPropertyPath());
+  }
+
 }