gistfile1.txt

// Add to serializer
if ([mapping isKindOfClass:[RKEntityMapping class]]) {
    NSAttributeDescription *attribute = [(RKEntityMapping *)mapping attributesByName:self.sourceKeyPath];
    if ([attribute attributeType] == NSBooleanAttributeType) {
        transformedValue = [value boolValue] ? @"true" : @"false";
    }
}