帶NSPredicate的NSFetchRequest不返回任何結果

Question

以下NSFetchRequest不會返回任何結果到數組中。我懷疑NSPredicate的語法有問題。有任何想法嗎？

-(NSManagedObject*)requestTheSingleEntity:(NSString *)entityDescription ForWhichIntegerAttribute:(NSString *)attribute isEqualTo:(int)value 
{ 
    NSFetchRequest *request = [[NSFetchRequest alloc]init]; 
    NSEntityDescription *e = [[model entitiesByName] objectForKey:entityDescription]; 

    [request setEntity:e]; 
    NSPredicate *p = [NSPredicate predicateWithFormat:@"(%@ == %@)", attribute, [NSNumber numberWithInt:value]]; 
    [request setPredicate:p]; 

    NSError *er; 
    NSArray *results = [context executeFetchRequest:request error:&er]; 
    NSLog(@"the count of results = %d", [results count]); 

Answer 1

你必須使用%K的屬性，而不是%@：

NSPredicate *p = [NSPredicate predicateWithFormat:@"%K == %@", attribute, [NSNumber numberWithInt:value]];