Mantle Objective C: mapping nested properties to JSON

I want to use Mantle to serialize some objects to this JSON:

{
"name": "John Smith",
"age": 30,
"department_id":123
}

I have two classes Department Employee:

#import <Mantle/Mantle.h>

    @interface Department : MTLModel <MTLJSONSerializing>

    @property(nonatomic)int id;
    @property(nonatomic)NSString *name;

    @end

and the Employee class:

#import <Mantle/Mantle.h>
#import "Department.h"

    @interface Employee : MTLModel <MTLJSONSerializing>

    @property(nonatomic)NSString *name;
    @property(nonatomic)int age;
    @property(nonatomic)Department *department;

    @end

@implementation Employee
+ (NSDictionary *)JSONKeyPathsByPropertyKey {

    return @{
             @"name":@"name",
             @"age":@"age",
             @"department.id":@"department_id"
             };
}
@end

when serializing an Employee instance I receive the following exception: "NSInternalInconsistencyException", "department.id is not a property of Employee."

What's wrong here? is there a way to serialize the object as as a single dictionary instead of nesting the department object inside the employee object?

OK, I got it from here: Mantle property class based on another property?

I modified the mapping dictionary to be like this

+ (NSDictionary *)JSONKeyPathsByPropertyKey {

    return @{
             @"name":@"name",
             @"age":@"age",
             NSStringFromSelector(@selector(department)) : @[@"department_id"]
             };
}

and added:

    + (NSValueTransformer *)departmentJSONTransformer {
        return [MTLValueTransformer transformerUsingReversibleBlock:^id(Department *department, BOOL *success, NSError *__autoreleasing *error) {
           return [MTLJSONAdapter JSONDictionaryFromModel:department error:nil];
        }];

}