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@JsonInclude(JsonInclude.Include.NON_NULL)
public class TestJson implements Serializable{
...
...
...
}

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@RunWith(SpringRunner.class)
@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
public class UserRepositoryTests {

    @Autowired
    private TestEntityManager entityManager;

    @Autowired
    private UserRepository repository;

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    //user_id_seq is the sequence that defined in posgresql database for id column
    
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "user_id_seq")
    @SequenceGenerator(name = "user_id_seq", sequenceName = "user_id_seq")
    @Column(name = "id")
    public int getId() {
        return id;
    }

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@Entity
public class User{

  .
  .
  .
  @PreUpdate
  public void preUpdate(){
      System.out.println("pre update executed");

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@SpringBootTest(classes = XXXApplication.class) //if we change Application.class we must declare here what the application main class is
@RunWith(SpringRunner.class)
public class UserControllerTests {

    private MockMvc mockMvc;

    @Autowired
    WebApplicationContext webApplicationContext;

    @Before

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@Entity
@Where(clause = "is_deleted=0")
public class Customer extends AbstractEntity implements Serializable{

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//first prepare pojo for query

@JsonInclude(JsonInclude.Include.NON_NULL)
public class UserResponseVM implements Serializable{

    @JsonProperty("_id")
    private UUID id;
    private String name;
    
    .

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@Configuration
@EnableWebMvc
public class WebConfig extends WebMvcConfigurerAdapter{

    @Override
    public void addCorsMappings(CorsRegistry registry) {
        registry.addMapping("/**")
                .allowedOrigins("*") //access from all origin
                .exposedHeaders("Authorization"); //for reading Authorization header

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//If you have auto generated field and it is not primary key. So you need below mapping for that column:

@Basic
@Column(name = "id", insertable = false)
public long getId() {
  return id;
}

//on above the important point is "insertable=false" statement
//if you use insertable=false db will generate and insert on its own to that column

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//to accomplish this problem  we need to add "updatable=false" to that column
//because without this definition when we need update a row automatically hibernate assign default value 0 (zero)

    @Basic
    @GeneratedValue
    @Column(name = "id", insertable = false, updatable = false)
    public long getId() {
        return id;
    }