我做了很多研究，但仍然无法弄清楚那里有什么问题。我的关系如下：
@Entity
@Table(name = "PARENT")
public class Parent implements Serializable{
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
@OneToMany(cascade = CascadeType.ALL)
@JoinColumn(name = "PARENT_ID", nullable = false)
private Set<Child> medicalHistories = new HashSet<>();
...
@Entity
@Table(name = "CHILD")
public class Child implements Serializable{
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int id;
父类有一个addChild（子子）方法。我首先实例化瞬态Parent，然后添加一些Childs。
当我尝试持久保存Parent对象时，抛出异常：
com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'PARENT_ID' cannot be null
sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
java.lang.reflect.Constructor.newInstance(Constructor.java:422)
com.mysql.jdbc.Util.handleNewInstance(Util.java:408)
com.mysql.jdbc.Util.getInstance(Util.java:383)
com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1049)
com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4226)
com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:4158)
com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2615)
com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2776)
com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2840)
com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2082)
com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2334)
com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2262)
com.mysql.jdbc.PreparedStatement.executeUpdate(PreparedStatement.java:2246)
org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:208)
org.hibernate.id.IdentityGenerator$GetGeneratedKeysDelegate.executeAndExtract(IdentityGenerator.java:96)
org.hibernate.id.insert.AbstractReturningDelegate.performInsert(AbstractReturningDelegate.java:58)
org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3032)
org.hibernate.persister.entity.AbstractEntityPersister.insert(AbstractEntityPersister.java:3558)
org.hibernate.action.internal.EntityIdentityInsertAction.execute(EntityIdentityInsertAction.java:98)
org.hibernate.engine.spi.ActionQueue.execute(ActionQueue.java:490)
org.hibernate.engine.spi.ActionQueue.addResolvedEntityInsertAction(ActionQueue.java:195)
org.hibernate.engine.spi.ActionQueue.addInsertAction(ActionQueue.java:179)
org.hibernate.engine.spi.ActionQueue.addAction(ActionQueue.java:214)
org.hibernate.event.internal.AbstractSaveEventListener.addInsertAction(AbstractSaveEventListener.java:324)
org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:288)
org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:194)
org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:125)
org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:206)
org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:149)
org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:801)
org.hibernate.internal.SessionImpl.persist(SessionImpl.java:794)
org.hibernate.engine.spi.CascadingActions$7.cascade(CascadingActions.java:314)
org.hibernate.engine.internal.Cascade.cascadeToOne(Cascade.java:350)
org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:293)
org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:161)
org.hibernate.engine.internal.Cascade.cascadeCollectionElements(Cascade.java:379)
org.hibernate.engine.internal.Cascade.cascadeCollection(Cascade.java:319)
org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:296)
org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:161)
org.hibernate.engine.internal.Cascade.cascade(Cascade.java:118)
org.hibernate.event.internal.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:460)
org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:294)
org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:194)
org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:125)
org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:206)
org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:149)
org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:75)
org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:811)
org.hibernate.internal.SessionImpl.persist(SessionImpl.java:784)
org.hibernate.internal.SessionImpl.persist(SessionImpl.java:789)
感谢任何帮助。