NSPredicate具有动态键和值 [英] NSPredicate with dynamic key and value

问题描述

我试图创建一个函数来搜索我的核心数据栈，并返回所需的键/值的消息量。

I am trying to create a function that searches through my core data stack and return the amount of messages for a desired key/value.

这里是我的代码：

NSFetchRequest      *messagesCountRequest = [NSFetchRequest fetchRequestWithEntityName:@"Message"];
NSEntityDescription *messageCountModel    = [NSEntityDescription entityForName:@"Message" inManagedObjectContext:_mainManagedObjectContext];

[messagesCountRequest setEntity:messageCountModel];

NSPredicate *messageCountPredicate = [NSPredicate predicateWithFormat:@"%@ == %@", key, value];

[messagesCountRequest setPredicate:messageCountPredicate];

count = (int)[_mainManagedObjectContext countForFetchRequest:messagesCountRequest error:nil];

问题是每次都返回0。我找到了问题的根源。当有静态键时，谓词看起来像这样。

The problem is that it returns 0 every time. I have located the source of the problem. When have a static key, the predicate looks like this.

key == "value"

当我通过动态键时，它看起来像这样

When I pass through a dynamic key, it looks like this

"key" == "value"

通过将NSString传递给谓词来放置的第一组双引号。我将如何解决这个问题？

So the problem appears to be the first set of double quotes around the key that is placed by passing a NSString to the predicate. How would I fix this?

编辑：为了清楚起见，我需要它像第一种情况，这是一个工作。

For clarity, I need it to be like the first case, that is the one that works.

推荐答案

要在谓词字符串中替换键，必须使用％K ，而不是％@ ：

To substitute a key in the predicate string, you have to use %K, not %@:

[NSPredicate predicateWithFormat:@"%K == %@", key, value];

从谓词格式字符串语法文档：

• ％@是对象值的var arg替换，通常是字符串，数字或日期。


• ％K是关键路径的var arg替换。 li>
  

• %@ is a var arg substitution for an object value—often a string, number, or date.
• %K is a var arg substitution for a key path.

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