// ------------------- ТЕСТ -------------------
@Test
void testUpdate() {
        Comment updatedComment = new Comment(...);
        commentRepository.save(updatedComment);

        Comment actual = commentRepository.findOne(updatedComment.getCommentId());

        assertThat(actual).isEqualToIgnoringGivenFields(updatedComment, "book"); // игнорируем book, чтобы программа не вошла в бесконечный цикл book-comment-book-comment-...

       // до сюда всё хорошо, теперь отдельно проверяем book, игнорируя comments 
        assertThat(actual.getBook()).isEqualToIgnoringGivenFields(updatedComment.getBook(), "comments"); // здесь ошибка Cannot locate field $$_hibernate_interceptor on class model.Book
    }

// ------------------- КНИЖКА -------------------
@Entity
@Table(name = "book")
@Data
@NoArgsConstructor
public class Book {
    @Id
    @GeneratedValue(strategy = IDENTITY)
    private Long bookId;

    private String title;
    private Integer year;

    @ManyToOne
    @JoinColumn(name = "author_id", nullable = false)
    private Author author;
    @ManyToOne
    @JoinColumn(name = "genre_id", nullable = false)
    private Genre genre;

    @OneToMany(
            mappedBy = "book",
            cascade = CascadeType.ALL,
            orphanRemoval = true
    )
    private List<Comment> comments = new ArrayList<>();
    // конструктор...
}

// ------------------- КОММЕНТ -------------------
@Entity
@Table(name = "comment")
@Data
@NoArgsConstructor
public class Comment {
    @Id
    @GeneratedValue(strategy = IDENTITY)
    private Long commentId;

    private String commentator;
    private String text;
    private LocalDateTime date;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "book_id", nullable = false)
    private Book book;

    // конструктор...
}