@Data
@Entity
public class Dojo {

    private @Id @GeneratedValue Long id;
    private String name;
    private String location;
    private Date created;
    @OneToMany(mappedBy = "dojo")
    @JsonIgnore
    private List<Workshop> workshops;

    private Dojo() {}

    public Dojo(String name, String location) {
        this.name = name;
        this.location = location;
        this.created = new Date();
        this.workshops = new ArrayList<>();
    }
//getters and setters ...
}

@Data
@Entity
public class Workshop {

    private @Id @GeneratedValue Long id;
    private String name;
    @ManyToOne
    private Dojo dojo;

    private Workshop() {}

    public Workshop(String name, Dojo dojo) {
        this.name = name;
        this.dojo = dojo;
    }
}

public interface WorkshopRepository extends CrudRepository<Workshop, Long> {}

@Test
public void testUpdateWorkshop() throws Exception {

    final String DOJO_NAME="My Dojo";
    final String DOJO_LOCATION="Liege";
    final String WORKSHOP_NAME="Stuff";
    final String HOST_PORT="http://localhost:8080";

     //creation of a dojo
    final Dojo DOJO = dojoRep.save(new Dojo(DOJO_NAME,DOJO_LOCATION));
     //creation of a workshop
    Workshop workshop = workshopRep.save(new Workshop(WORKSHOP_NAME,DOJO));

    String newValue = "After Test";

    System.out.println("before update");
    System.out.println(workshop.getName()+" == "+WORKSHOP_NAME);

    Long oldID = workshop.getId();

    //As you can see I didn't modify the workshop object
    HttpEntity<Workshop> entity = new HttpEntity<Workshop>(workshop);
    ResponseEntity<String> response = template.exchange(HOST_PORT+"/api/workshops/"+oldID, HttpMethod.PUT, entity, String.class, oldID);

    assert response.getStatusCodeValue() == 200;

    //re-Get the updated workshop
    workshop = workshopRep.findOne(oldID);

    System.out.println("after update");
    System.out.println(workshop.getName()+" == "+WORKSHOP_NAME);

    // as I didn't set the newValue, it must fail and workshop.getName() must stay equal to "Stuff".
    Assert.assertEquals("Update does not work",newValue,workshop.getName());
}

before update
Stuff == Stuff

after update
My Dojo == Stuff

Failed tests:
  WorkshopTests.testUpdateWorkshop:218 Update not work expected:<[After Test]> but was:<[My Dojo]>

mockMvc.perform(put(HOST_PORT+"/api/workshops/"+oldID)
                .contentType(MediaType.APPLICATION_JSON_UTF8)
                .content(convertObjectToJsonBytes(workshop))
);

private byte[] convertObjectToJsonBytes(Object object) throws IOException {
    ObjectMapper mapper = new ObjectMapper();
    System.out.println("log my face ");
    System.out.println(mapper.writeValueAsString(object));
    return mapper.writeValueAsBytes(object);
}

{"id":1,"name":"Stuff","dojo":{"id":1,"name":"My Dojo","location":"Liege","created":1500799092330}}

{
  "name" : "Stuff",
  "_links" : {
    "self" : {
      "href" : "http://localhost-core:8080/api/workshops/1"
    },
    "workshop" : {
      "href" : "http://localhost-core:8080/api/workshops/1"
    },
    "dojo" : {
      "href" : "http://localhost-core:8080/api/workshops/1/dojo"
    }
  }
}