ModelQuery: throw an error when fed with nonexistent relation

src/db/ModelQuery.ts

@@ -183,6 +183,7 @@ export default class ModelQuery<M extends Model> {
 
             // Eager loading init map
             for (const relation of this.relations) {
+                if (model[relation] === undefined) throw new Error(`Relation ${relation} doesn't exist on ${model.constructor.name}.`);
                 relationMap[relation].push(model[relation]);
             }
         }