ios - 检查以确保核心数据中的实体属性在保存时不相同

Question

我有一个正在使用核心数据的应用程序。我的应用程序能够添加新实体并填写该实体的“名称”属性。我不确定如何做是确保不存在使用相同“名称”属性创建的 2 个实体。我读过这篇关于独特约束的文章( Constraints in Core Data )，它似乎有点高于我的水平。所以我试图获取页面顶部的代码并实现它。这是我所拥有的:

// check to see if the name is already saved
        // check to see if the name is already saved
        let fetchRequest = NSFetchRequest(entityName: "Person")

        fetchRequest.resultType = .ManagedObjectIDResultType

        let predicate = NSPredicate(format: "name = %@ ", name)

        let results = coreDataStack.context.executeFetchRequest(fetchRequest)

        if (results.name == personName.text) {
            //don't use it!
        } else {
            //create it!
        }

我不确定的部分是 NSPredicate，我想查看我对在每个 Person 实体上找到的“name”属性的提取请求。

Answer 1

这是我最终使用的

// check to see if the name is already saved
        let fetchRequest = NSFetchRequest(entityName: "Person")

        fetchRequest.resultType = .CountResultType
        // check the name attribute against the person name input
        let predicate = NSPredicate(format: "name = %@ ", personName.text!)

        fetchRequest.predicate = predicate;


        do{
            let results = try coreDataStack.context.executeFetchRequest(fetchRequest) as! [NSNumber]
            let count = results.first!.integerValue

            if (count == 0) {
                //use it!
                print("You can use that name") 
            }else{
               // don't use it!
               print("You already have that name in use")
            }
        } catch let error as NSError {
            print("Could not fetch \(error), \(error.userInfo)")
        }