NSPredicate predicateWithFormat传递属性名称 [英] NSPredicate predicateWithFormat passing in name of attribute

问题描述

关于NSPredicate的简单问题。我试图用如下所示的传递值构造我的谓词：

Simple question regarding NSPredicate's. I'm trying to construct my predicate with "passed in" values like so :

NSPredicate* currentPredicate = [NSPredicate predicateWithFormat:@"%@ == %@",key,[changesDict valueForKey:@"Id"] ];

但是，我无法正常工作。如果我插入实际值，我通过它的确工作，但。所以这工作：

However, I haven't been able to get this to work correctly. If I insert the actual value I pass through it does work though. So this works :

NSPredicate* currentPredicate = [NSPredicate predicateWithFormat:@"contactId == %@",[changesDict valueForKey:@"Id"] ];

（注意，我插入了contactId，而不是前面的例子，

(Notice i inserted contactId as opposed to the previous example where I pass a string by the same name)

要排除故障我NSLogged两个谓词查看他们的描述，他们是不同的。我会在下面显示。

To troubleshoot I NSLogged the two predicates looking at their descriptions and they were different. I'll show them below.

这是工作中的。

2013-01-17 10:29:25.513 TestingCoreData[1776:3b03] contactId == "5878"

这是不工作的。

2013-01-17 10:29:25.513 TestingCoreData[1776:3b03] "contactId" == "5878"

所以我可以看到插入一个字符串，其中我真正想要的属性的名称，我稍后将在获取请求中使用。但是有什么方法可以通过传递值来实现这一点。

So I can kind of see that it's inserting a string where I really just want the name of the attribute that i'll later be using in the fetch request. But is there any way to accomplish this by passing in values?

推荐答案

>关键路径，您必须使用％K 格式：

[NSPredicate predicateWithFormat:@"%K == %@", key, [changesDict valueForKey:@"Id"]];

（请参阅解析器基础。）

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