我正在尝试使用NSPredicate查询表.这本质上是我在做什么
NSNumber *value = [NSNumber numberWithInteger: 2]; NSString *columnName = @"something_id"; NSLog(@"%@ == %@",columnName,value); NSPredicate *refQuery = [NSPredicate predicateWithFormat: @"%@ == %@",value];
NSLog打印出我的期望(“something_id == 2”),但谓词不起作用.
但是,如果我将更改为：
NSPredicate *refQuery = [NSPredicate predicateWithFormat: @"something_id == %@",value];
那么为什么这不工作,我该怎么解决呢？
解决方法
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.
所以,在你的情况下,你需要把％K作为一个关键字到columnName,而不是％@,它将被加上引号：
NSPredicate *refQuery = [NSPredicate predicateWithFormat: @"%K == %@",value];
希望这清楚你的疑惑.
