OneModelRelation: add set(), clear() methods

src/db/ModelRelation.ts

@@ -113,6 +113,13 @@ export class OneModelRelation<S extends Model, O extends Model> extends ModelRel
         this.cachedModels = models.filter(m => m[this.dbProperties.foreignKey] === this.getModelID())[0] || null;
     }
 
+    public async set(model: O): Promise<void> {
+        (this.model as Model)[this.dbProperties.localKey] = model[this.dbProperties.foreignKey];
+    }
+
+    public async clear(): Promise<void> {
+        (this.model as Model)[this.dbProperties.localKey] = undefined;
+    }
 }
 
 export class ManyModelRelation<S extends Model, O extends Model> extends ModelRelation<S, O, O[]> {