Spring data rest jpa post object with existing related entity - spring-data-rest

Question

I have two entities
#Entity
public class ForwardRule {
#Column(nullable = false)
private String identifier;
#OneToOne
#JoinColumn(name = "user_id")
private User user;
}
#Entity
public class User {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long userId;
#Column(nullable = false)
private String identifier;
}
For both I have Repositories annotated with: #RepositoryRestResource
I have an existing user and I want to post a FowardRule with an existing User.
String requestJson = "{" +
"\"identifier\": \"6bff6eb0-9b47-42fd-8ac6-339a93aa310d\"," +
"\"user\" : {\"href\":\"http://localhost:9000/users/1\"}"+
"}";
HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON);
HttpEntity<String> entity = new HttpEntity<>(requestJson, headers);
String answer = restTemplate.postForObject("http://localhost:9000/forwardRules", entity, String.class);
System.out.println(answer);
As a result I get a mysql exception: Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'user_id' cannot be null
I would like to know two things:
why is the user_id null
how can I do the post with Java objects? In following style. This works for getting and posting simple objects.
ForwardRuleView newOne = new ForwardRuleView();
newOne.setUser(existingUser);
newOne.setIdentifier("howie");
ResponseEntity forwardRuleViewResponseEntity = new RestTemplate().postForEntity("http://localhost:9000/forwardRules", newOne, ForwardRuleView.class);
Update
Json post can be done with:
String requestJson = "{" +
"\"identifier\": \"6bff6eb0-9b47-42fd-8ac6-339a93aa310d\"," +
"\"user\" : {"http://localhost:9000/users/1\"}"+
"}";
Only need to know how I can post it with Pojo's