add UserGroupRelation.findAllUserGroupIdsRelatedToUser method

src/server/models/user-group-relation.js

@@ -128,6 +128,21 @@ class UserGroupRelation {
       });
   }
 
+  /**
+   * find all UserGroup IDs that related to specified User
+   *
+   * @static
+   * @param {User} user
+   * @returns {Promise<ObjectId[]>}
+   */
+  static findAllUserGroupIdsRelatedToUser(user) {
+    return this
+      .find({ relatedUser: user.id })
+      .map(relation => {
+        return relation.relatedGroup._id;
+      });
+  }
+
   /**
    * find all entities with pagination
    *