核心数据谓词不起作用

本文介绍了核心数据谓词不起作用的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我是Mac的新手，正在使用Core Data编写我的第一个应用程序。我有一个数据模型，其中有一个实体SizeRecord，它具有单个属性 size和单个关系 files，后者是一组FileRecord引用。 我能够创建一个SizeRecord并将其正确保存到数据库中。我还能够从数据库中获取记录：

I am new to Mac and am writing my first app using Core Data. I have a data model in which there is entity SizeRecord which has single attribute "size" and single relationship "files" which is a set of FileRecord references. I am able to create a SizeRecord and properly save it into the database. I am also able to obtain the record from the database:

let sizeFetch: NSFetchRequest<SizeRecord> = SizeRecord.fetchRequest() let size: UInt64 = 106594 //sizeFetch.predicate = NSPredicate(format: "size == %i", size) do { let fetchedSizes = try dbCtx.ctx.fetch(sizeFetch) } catch { }

我运行此命令时，fetchedSizes数组按预期包含单个读取的记录：

Wen I run this, fetchedSizes array contains single fetched record as expected:

▿ 1 element - 0 : <SizeRecord: 0x10584fcc0> (entity: SizeRecord; id: 0x100495ea0 <x-coredata://F7EC8A0C-06E1-46AE-A875-34E4523AAC2A/SizeRecord/p1> ; data: { files = ( "0x100336310 <x-coredata://F7EC8A0C-06E1-46AE-A875-34E4523AAC2A/FileRecord/p2>" ); size = 106594;

但是，当我取消注释上面的谓词时，结果是一个空数组。这是sizeFetch的内容：

however when I uncomment the predicate above, I get an empty array as a result. This is a content of sizeFetch:

<NSFetchRequest: 0x1058e4890> (entity: SizeRecord; predicate: (SIZE == 106594); sortDescriptors: ((null)); type: NSManagedObjectResultType; )

请问我在做什么错吗？一定很简单很愚蠢：）

Please any idea what I am doing wrong? It must be something very simple and stupid :)

推荐答案

SIZE是谓词格式字符串语法中的保留字，参见谓词格式字符串语法 e谓词编程指南。

"SIZE" is a reserved word in the predicate format string syntax, see Predicate Format String Syntax in the "Predicate Programming Guide".

解决方案是使用％K 关键字扩展（通常为a避免与保留字冲突的好主意）：

The solution is to use the %K keyword expansion (which is generally a good idea to avoid conflicts with reserved words):

NSPredicate(format: "%K == %i", "size", size)

，甚至使用 #keyPath code>指令，可让编译器 check 属性名称：

or even better with the #keyPath directive which allows the compiler to check the property name:

NSPredicate(format: "%K == %i", #keyPath(SizeRecord.size), size)