Почему происходит ошибка Lazy-инициализации у энтити?

@Entity
@Table(name = "train")
public class Train {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @Column(name = "name")
    private String name;

    @OneToMany(mappedBy = "train", cascade = CascadeType.ALL)
    private Set<Seat> seats;
    ...
    // Геттеры и сеттеры

@Entity
@Table(name = "seat")
public class Seat {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Long id;

    @Column(name = "carriage")
    private Integer carriage;

    @Column(name = "seat")
    private Integer seat;

    @ManyToOne(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    @JoinColumn(name = "train_id", nullable = false)
    private Train train;
    ...
    // Другой код

org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.slandshow.models.Train.seats, could not initialize proxy - no Session