New activity gets its case from the url

modules/opencase_entities/src/Entity/OCActivity.php

@@ -74,6 +74,13 @@ class OCActivity extends RevisionableContentEntityBase implements OCActivityInte
 
   use EntityChangedTrait;
 
+  /**
+   * When creating an activity, it sets the case id from the URL.
+   */
+  public static function defaultVal() {
+    return \Drupal::request()->query->get('case_id');;
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -267,6 +274,7 @@ class OCActivity extends RevisionableContentEntityBase implements OCActivityInte
           'placeholder' => '',
         ],
       ])
+      ->setDefaultValueCallback('Drupal\opencase_entities\Entity\OCActivity::defaultVal')
       ->setRequired(TRUE);
 
     $fields['status'] = BaseFieldDefinition::create('boolean')

modules/opencase_entities/src/Entity/OCCase.php

@@ -72,6 +72,7 @@ use Drupal\user\UserInterface;
  */
 class OCCase extends RevisionableContentEntityBase implements OCCaseInterface {
 
+  use EntityChangedTrait;
 
   /**
    * When creating a case, it sets the first involved party to the actor
@@ -81,8 +82,6 @@ class OCCase extends RevisionableContentEntityBase implements OCCaseInterface {
     return \Drupal::request()->query->get('actor_id');;
   }
 
-  use EntityChangedTrait;
-
   /**
    * {@inheritdoc}
    */