Only schedule notification for standard newsletters

[MAILPOET-1571]

lib/Models/StatsNotification.php

@@ -5,5 +5,29 @@ namespace MailPoet\Models;
 class StatsNotification extends Model {
   public static $_table = MP_STATS_NOTIFICATIONS_TABLE;
 
+  /** @return StatsNotification */
+  static function createOrUpdate($data = array()) {
+    $model = null;
+
+    if(isset($data['id']) && (int)$data['id'] > 0) {
+      $model = static::findOne((int)$data['id']);
+    }
+
+    if(!$model && isset($data['task_id']) && $data['newsletter_id']) {
+      $model = self::where('newsletter_id', $data['newsletter_id'])
+        ->where('task_id', $data['task_id'])
+        ->findOne();
+    }
+
+    if(!$model) {
+      $model = static::create();
+      $model->hydrate($data);
+    } else {
+      unset($data['id']);
+      $model->set($data);
+    }
+
+    return $model->save();
+  }
 
 }