Hibernate和JPA:如何对String进行外键约束 [英] Hibernate and JPA: how to make a foreign key constraint on a String

问题描述

我正在使用Hibernate和JPA.如果我有两个简单的实体:

I am using Hibernate and JPA. If I have two simple entities:

@Entity
@Table(name = "container")
public class Container {
  @Id
  @Column(name="guid")
  private String guid;
}

@Entity
@Table(name="item")
public class Item {
  @Id
  @Column(name="guid")
  private String guid;

  @Column(name="container_guid")
  private String containerGuid;
}

，并且我想确保如果引用的容器不存在，则无法插入项目.我希望不要在项目对象(ManyToOne)中填充一个Container对象，如果可以的话我该怎么办?

and I want to insure that inserting an Item fails if the referenced Container does not exist. I would prefer not to have a Container object populated inside the item object (ManyToOne), how would I do this if it is possible to do?

推荐答案

您可以使用columnDefinition属性声明任意约束:

You can declare arbitrary constraint using columnDefinition attribute:

@Column(name="container_guid", 
     columnDefinition = "VARCHAR(255) REFERENCES container(guid)")
private String containerGuid;

但是请注意，Hibernate对这个约束一无所知，因此，例如，它可能不会按照正确的顺序执行插入操作，等等.

Note, however, that Hibernate doesn't know anything about this constraint, so that, for example, it may not perform inserts in proper order with respect of it and so on.

因此，最好创建一个@ManyToOne关系.如果您担心设置此属性需要对Container进行额外的SQL查询，则可以使用Session.load()/EntityManager.getReference()获取代理，而无需发出实际查询.

Therefore it would be better to create a @ManyToOne relationship. If you are afraid of extra SQL query for Container needed to set this property, you can use Session.load()/EntityManager.getReference() to get a proxy without issuing actulal query.

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