objective-c - 列名的 NSPredicate 问题

Question

我正在尝试使用 NSPredicate 查询表。这基本上是我正在做的事情:

NSNumber *value = [NSNumber numberWithInteger: 2];
NSString *columnName = @"something_id";

NSLog(@"%@ == %@", columnName, value);
NSPredicate *refQuery = [NSPredicate predicateWithFormat: @"%@ == %@", columnName, value];

NSLog 打印出我所期望的(“something_id == 2”)，但谓词不起作用。 但是，如果我将谓词更改为:

NSPredicate *refQuery = [NSPredicate predicateWithFormat: @"something_id == %@", value];

那么为什么这行不通，我该如何解决呢？

Answer 1

The Predicate Programming Guide from Apple说:

%@ 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.

When string variables are substituted into a format string using %@ , they are surrounded by quotation marks. If you want to specify a dynamic property name, use %K in the format string.

因此，在您的情况下，您需要将 %K 作为 columnName 的键路径，而不是 %@，后者将添加引号:

NSPredicate *refQuery = [NSPredicate predicateWithFormat: @"%K == %@", columnName, value];

希望这能消除您的疑虑。