核心数据获取特定对象

Question

- (NSManagedObjectContext *)anObjectByEntityForName:(NSString *)entityName withValue:(NSObject *)value forKeyPath:(NSString *)keyPath {
NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:entityName inManagedObjectContext:self.managedObjectContext]];
[request setPredicate:[NSPredicate predicateWithFormat:@"%@ == %@", keyPath, value]];

NSError *error = nil;
NSMutableArray *mutableFetchResults = [[self.managedObjectContext executeFetchRequest:request error:&error] mutableCopy];
if (!mutableFetchResults) {
    [request release];
    [mutableFetchResults release];
    return nil;
}

if ([mutableFetchResults count] == 0) {
    [request release];
    [mutableFetchResults release];
    return nil;
}

id anObject = [mutableFetchResults objectAtIndex:0];
[request release];
[mutableFetchResults release];

return anObject;
}

对于键路径“isSelected”和值@YES，此代码返回nil。但如果没有谓词，则返回所有对象。在数据库中至少有 1 个符合条件的对象。 whit 有什么问题？

Answer 1

对于键或键路径，必须使用 %K 格式：

[request setPredicate:[NSPredicate predicateWithFormat:@"%K == %@", keyPath, value]];

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