Accessing the ALAssetsGroup information for an ALAsset object

Question

It's common to enumerate the ALAssetsGroup to get the ALAsset objects, like this way:

[assetGroup enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
    NSLog(@"%@",result);              
}];

But I have a special use case, which is accessing the ALAssetsGroup information for an specific ALAsset object. How to do this by avoid enumerating the whole library? Thanks!