@interface LoginRequest : NSObject
{
    NSString* _deviceId;
}

@property (nonatomic, retain) NSString* deviceId;

@end

objectManager.serializationMIMEType = RKMIMETypeFormURLEncoded;

        RKObjectMapping* pmsg = [RKObjectMapping mappingForClass:[NSMutableDictionary class]];
        [pmsg mapKeyPath: @"deviceId" toAttribute:@"DeviceId"];

        RKObjectMapping* pmsgSerializeMapping = [pmsg inverseMapping];

        [objectManager.mappingProvider setSerializationMapping:pmsgSerializeMapping forClass:[LoginRequest class]];

        [objectManager.router routeClass:[LoginRequest class] toResourcePath:@"/login" forMethod:RKRequestMethodPOST];

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<LoginRequest 0x8c9c980> valueForUndefinedKey:]: this class is not key value coding-compliant for the key DeviceId.'

RKObjectManager *objectManager = [RKObjectManager sharedManager];

LoginRequest* request = [LoginRequest alloc];
request.deviceId = @"Test";

[objectManager postObject:request delegate: self];

RKObjectMapping* pmsg = [RKObjectMapping mappingForClass:[NSMutableDictionary class]];
        [pmsg mapAttributes:@"DeviceId", nil];

        [objectManager.mappingProvider setSerializationMapping:pmsg forClass:[LoginRequest class]];

        [objectManager.router routeClass:[LoginRequest class] toResourcePath:@"/login" forMethod:RKRequestMethodPOST];