NSPredicate格式字符串不工作

问题描述

在我的代码中，我想检查一个记录是否已经存在，所以我知道是否创建它或更新它。但我遇到了一个问题。问题是当我使用这个：

In my code, I want to check and see if a record already exists, so I know whether to create it or update it. But I ran into a problem. The problem is when I use this:

NSPredicate *pred = [NSPredicate predicateWithFormat:@"%@ == %@", ATTRIBUTE_ID, idNumber];
[request setPredicate:pred];

这不起作用。它总是声称没有找到结果。然而，它工作正常，当我重写如下：

This doesn't work. It always claims no results were found. However, it works just fine when I rewrite it like so:

NSExpression *lhs = [NSExpression expressionForKeyPath:ATTRIBUTE_ID];
NSExpression *rhs = [NSExpression expressionForConstantValue:idNumber];
NSPredicate *pred = [NSComparisonPredicate
                                     predicateWithLeftExpression:lhs
                                     rightExpression:rhs
                                     modifier:NSDirectPredicateModifier
                                     type:NSEqualToPredicateOperatorType
                                     options:0];
[request setPredicate:pred];

我在格式字符串中丢失或做错了什么？

What am I missing or doing incorrectly in the format string?

ATTRIBUTE_ID 是一个键，所以你应该使用％K

Your ATTRIBUTE_ID is a key, so you should use %K in the format string for that part of it.

格式字符串看起来像这样（注释中指出的大写K）：

The format string would look like this (upper case K as pointed out in the comments):

[NSPredicate predicateWithFormat:@"%K == %@", ATTRIBUTE_ID, idNumber];

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