java-Spring-Data-Jpa保存：保存为null的相关实体不是

我有实体时间表：

@Entity @Table(name = TableUtils.TIMETABLE)public class Timetable {

private static final long serialVersionUID = -1307879048598194633L;

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = ColumnUtils.ID)
private long id;

@OneToMany(mappedBy = "timetable",cascade = {CascadeType.REMOVE,CascadeType.PERSIST})
private List<TimetableCell> timetableCells;

public List<TimetableCell> getTimetableCells() {
    return timetableCells;
}

public void setTimetableCells(List<TimetableCell> timetableCells) {
    this.timetableCells = timetableCells;
}
}

和一个实体的时间表单元：

@Entity
@Table(name = TableUtils.TIMETABLE_CELL)
public class TimetableCell extends AbstractElement {

private static final long serialVersionUID = -8083688091896353882L;

@ManyToOne(cascade = CascadeType.PERSIST)
@JoinColumn(name = ColumnUtils.WEEK_DAY_ID,nullable = false)
private WeekDay weekDay;

@ManyToOne(cascade = CascadeType.PERSIST)
@JoinColumn(name = ColumnUtils.LESSON_HOUR_ID,nullable = false)
private LessonHour lessonHour;

@ManyToOne(cascade = CascadeType.PERSIST)
@JoinColumn(name = ColumnUtils.TIMETABLE_ID,nullable = false)
private Timetable timetable;

...
}

首先,我使用weekDay和LessonHours创建时间表单元格.在将此列表设置为时间表并尝试保存之后.但是所有时间表单元格都使用空的weekDay和空的LessonHour保存.
我调试了在保存之前,还设置了所有字段.

timetableRepository.save(timetable);

调试结果显示如下