Clear tables in the correct order.

The table definitions have constraints between requests and panels.
Clearing the requests table first will violate those constraints.

Refs cakephp/docs#3163

Author: markstory

src/Model/Table/RequestsTable.php

@@ -79,17 +79,23 @@ public function gc()
         if (time() % 100 !== 0) {
             return;
         }
-        $query = $this->query()
+        $purge = $this->find()
+            ->select(['id'])
+            ->hydrate(false)
+            ->where(['requested_at <=' => new \DateTime('-2 weeks')])
+            ->extract('id')
+            ->toArray();
+
+        $query = $this->Panels->query()
             ->delete()
-            ->where(['requested_at <=' => new \DateTime('-2 weeks')]);
+            ->where(['request_id IN' => $purge]);
         $statement = $query->execute();
         $statement->closeCursor();
 
-        $existing = $this->find()->select(['id']);
-
-        $query = $this->Panels->query()
+        $query = $this->query()
             ->delete()
-            ->where(['request_id NOT IN' => $existing]);
+            ->where(['id IN' => $purge]);
+
         $statement = $query->execute();
         $statement->closeCursor();
     }