update

app/common/model/MaintenanceCategory.php

@@ -53,6 +53,6 @@ class MaintenanceCategory extends BaseModel
     }
 
     public function maintenances() {
-        return $this->hasMany(MaintenanceCategory::class,'category_id', 'id');
+        return $this->hasMany(Maintenance::class,'category_id', 'id');
     }
 }