Mapping an object to the resource with RestKit
===

I'm learning RestKit and would like to POST a payload using the object mapper. I'm using registerMapping: but this creates the mapping on a root key. Is there a way to map the object to the resource instead? My mapping code looks like this:

    [self.objectManager.router routeClass:[Session class] toResourcePath:@"/sessions" forMethod:RKRequestMethodPOST];
    [self.objectManager.mappingProvider registerMapping:[Session mappingForObjectStore:self.objectManager.objectStore] 
                                        withRootKeyPath:@""];
    
    [self.objectManager postObject:session usingBlock:^(RKObjectLoader *loader) {
        loader.resourcePath = @"/sessions";
        loader.delegate = self;

    }];

which creates a payload like this:

> {
"" : {data:"stuff"}
}

but I'm after a payload like this:

> {
data:"stuff"
}

What's The RestKit Way here?