Issue #2134967 by plopesc, swentel, chx: FieldDefinitionInterface should include a getTargetEntityTypeId().

core/lib/Drupal/Core/Entity/EntityManager.php

@@ -309,8 +309,12 @@ class EntityManager extends PluginManagerBase implements EntityManagerInterface
         $entity_type = $this->getDefinition($entity_type_id);
         $class = $entity_type->getClass();
 
+        $base_definitions = $class::baseFieldDefinitions($entity_type_id);
+        foreach ($base_definitions as &$base_definition) {
+          $base_definition->setTargetEntityTypeId($entity_type_id);
+        }
         $this->entityFieldInfo[$entity_type_id] = array(
-          'definitions' => $class::baseFieldDefinitions($entity_type_id),
+          'definitions' => $base_definitions,
           // Contains definitions of optional (per-bundle) fields.
           'optional' => array(),
           // An array keyed by bundle name containing the optional fields added

core/lib/Drupal/Core/Field/FieldDefinition.php

@@ -285,6 +285,27 @@ class FieldDefinition extends ListDefinition implements FieldDefinitionInterface
     return $this->getSetting('default_value');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getTargetEntityTypeId() {
+    return isset($this->definition['entity_type']) ? $this->definition['entity_type'] : NULL;
+  }
+
+  /**
+   * Sets the ID of the type of the entity this field is attached to.
+   *
+   * @param string $entity_type_id
+   *   The name of the target entity type to set.
+   *
+   * @return static
+   *   The object itself for chaining.
+   */
+  public function setTargetEntityTypeId($entity_type_id) {
+    $this->definition['entity_type'] = $entity_type_id;
+    return $this;
+  }
+
   /**
    * {@inheritdoc}
    */

core/lib/Drupal/Core/Field/FieldDefinitionInterface.php

@@ -255,6 +255,22 @@ interface FieldDefinitionInterface extends ListDefinitionInterface {
    */
   public function getDefaultValue(EntityInterface $entity);
 
+  /**
+   * Returns the ID of the type of the entity this field is attached to.
+   *
+   * This method should not be confused with EntityInterface::entityType()
+   * (configurable fields are config entities, and thus implement both
+   * interfaces):
+   *   - FieldDefinitionInterface::getTargetEntityTypeId() answers "as a field,
+   *     which entity type are you attached to?".
+   *   - EntityInterface::getEntityTypeId() answers "as a (config) entity, what
+   *     is your own entity type".
+   *
+   * @return string
+   *   The name of the entity type.
+   */
+  public function getTargetEntityTypeId();
+
   /**
    * Returns the field schema.
    *

core/modules/field/lib/Drupal/field/Entity/Field.php

@@ -641,6 +641,13 @@ class Field extends ConfigEntityBase implements FieldInterface {
     return array('type' => 'hidden');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getTargetEntityTypeId() {
+    return $this->entity_type;
+  }
+
   /**
    * {@inheritdoc}
    */

core/modules/field/lib/Drupal/field/Entity/FieldInstance.php

@@ -598,6 +598,13 @@ class FieldInstance extends ConfigEntityBase implements FieldInstanceInterface {
     return array('type' => 'hidden');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getTargetEntityTypeId() {
+    return $this->entity_type;
+  }
+
   /**
    * {@inheritdoc}
    */