Spring Boot - testing repository - Referential integrity constraint violation

I've got a problem that I was struggling for a while. I'm trying to test repository of my Spring Boot app, but unfortunelly when i was sure how to test repositories I stucked here getting an erorr "Naruszenie więzów integralności" which means "Referential integrity constraint violation". I thing I tried most of solutions that I found on the stack internet, but none of them words for me ;/. I have got an error like below:

17:42:20.639 [main] ERROR org.hibernate.engine.jdbc.spi.SqlExceptionHelper - Naruszenie więzów integralności: "FKKKTWEXQ2WLBTEVF2AXEQH8ANX: PUBLIC.POST FOREIGN KEY(POST_USER_ID) REFERENCES PUBLIC.USERS(USER_ID) (11)"
Referential integrity constraint violation: "FKKKTWEXQ2WLBTEVF2AXEQH8ANX: PUBLIC.POST FOREIGN KEY(POST_USER_ID) REFERENCES PUBLIC.USERS(USER_ID) (11)"; SQL statement:
insert into post (data, date, description, imagine_name, imagine_type, post_user_id, tittle, post_id) values (?, ?, ?, ?, ?, ?, ?, ?) [23506-200]
17:42:20.641 [main] DEBUG org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl - JDBC transaction marked for rollback-only (exception provided for stack trace)
java.lang.Exception: exception just for purpose of providing stack trace
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.markRollbackOnly(JdbcResourceLocalTransactionCoordinatorImpl.java:324)
    at org.hibernate.engine.transaction.internal.TransactionImpl.markRollbackOnly(TransactionImpl.java:203)
    at org.hibernate.internal.AbstractSharedSessionContract.markForRollbackOnly(AbstractSharedSessionContract.java:396)
    at org.hibernate.internal.ExceptionConverterImpl.handlePersistenceException(ExceptionConverterImpl.java:297)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:155)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:181)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:188)
    at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1364)
    at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1347)
    at org.springframework.boot.test.autoconfigure.orm.jpa.TestEntityManager.flush(TestEntityManager.java:171)
    at com.company.art.repository.PostRepositoryTests.findByPostUser_Id_One(PostRepositoryTests.java:45)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    ...

And that's my code:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes = ArtApplication.class, loader = AnnotationConfigContextLoader.class)
@DataJpaTest
public class PostRepositoryTests {
    @Autowired
    private TestEntityManager entityManager;

    @Autowired
    private PostRepository postRepository;

    @Test
    public void findByPostUser_Id_One(){
        //arrange
        Post post = new Post();
        User user = FakeDataSet.getFakeUser();
        post.setPostUser(user);
        Integer expected = 1;
        entityManager.merge(FakeDataSet.getFakeUser());
        entityManager.flush();

        entityManager.merge(post);
        entityManager.flush(); // <----- line with error
        //act
        List<Post> found = postRepository.findByPostUser_Id(FakeDataSet.getFakeUser().getId());
        //assert
        assertThat(found.size()).isEqualTo(expected);
    }
}

Repository:

@Repository
public interface PostRepository extends JpaRepository<Post, Integer> {
    List<Post> findByPostUser_Id(Integer id);
}

My fake data class:

public class FakeDataSet {
    public static User getFakeUser(){
        User user = new User();
        user.setId(11);
        user.setActive(true);
        user.setUserName("TheTest");
        user.setPassword("Password");
        user.setEmail("test@test.com");
        user.setName("Test");
        user.setLastName("Testing");
        return user;
    }

    public static Post getFakePost(){
        Post post = new Post();
        post.setTittle("Title");
        post.setId(1);
        post.setImagineType("image/png");
        post.setPostUser(FakeDataSet.getFakeUser());
        post.setData(new byte[5]);
        post.setImagineName("Image");
        post.setDate(Calendar.getInstance());
        post.setDescription("Description");

        return post;
    }
}

Model classes:

@AllArgsConstructor
@NoArgsConstructor
@Entity
@Table(name = "post")
public class Post {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "post_id")
    private Integer id;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "post_user_id")
    private User postUser;
    @CreationTimestamp
    @Temporal(TemporalType.TIMESTAMP)
    @Column(name = "date")
    private Calendar date;
    @Column(name = "imagine_name")
    private String imagineName;
    @Column(name = "imagine_type")
    private String imagineType;
    @Lob
    private byte[] data;
    @Column(name = "description", length = 500)
    @Length(max = 500)
    private String description;
    @Column(name = "tittle", length = 30)
    @Length(max = 30)
    private String tittle;
    @Nullable
    @OneToMany(mappedBy = "likedPost", cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<PostLikes> postLikes;
    @Transient
    private MultipartFile file;
}

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
@Entity
@Table(name = "users")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "user_id")
    private int id;
    @Column(name = "user_name")
    @Length(min = 5, message = "*Your user name must have at least 5 characters")
    @NotEmpty(message = "*Please provide a user name")
    private String userName;
    @Column(name = "email")
    @Email(message = "*Please provide a valid Email")
    @NotEmpty(message = "*Please provide an email")
    private String email;
    @Column(name = "password")
    @Length(min = 5, message = "*Your password must have at least 5 characters")
    @NotEmpty(message = "*Please provide your password")
    private String password;
    @Column(name = "name")
    @NotEmpty(message = "*Please provide your name")
    private String name;
    @Column(name = "last_name")
    @NotEmpty(message = "*Please provide your last name")
    private String lastName;
    @Column(name = "active")
    private Boolean active;
    @ManyToMany(cascade = CascadeType.MERGE)
    @JoinTable(name = "users_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
    private Set<Role> roles;
    @OneToMany(mappedBy = "postUser", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.LAZY)
    private Set<Post> posts;
    @OneToMany(mappedBy = "userThatLike", cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<PostLikes> postsLiked;
    @Column(name = "is_banned")
    private boolean isBanned;

}

Do you have any idea what I messed up? Thank for any tip! ;D