hibernate多对多中间表的操作
Intro

It is not straightforward to realise a many-to-many association with JPA when in the join table there is at least an extra column. In this small tutorial I’m going to show how to design entity objects that will handle the many-to-many relation and which annotations are needed in order to fix a redundancy that we will see in the solution adopted in the following wiki.

This tutorial is a mix up of different sources. The first solution I’m going to show is the one suggested in a wiki.

Mapping a Join Table with Additional Columns (in a JPA pure style)

Source: en.wikibooks.org

A frequent problem is that two classes have a ManyToMany relationship, but the relational join table has additional data. For example if Employee has a ManyToMany with Project but the PROJ_EMP join table also has an IS_TEAM_LEAD column. In this case the best solution is to create a class that models the join table. So an ProjectAssociation class would be created. It would have a ManyToOne to Employee and Project, and attributes for the additional data. Employee and Project would have a OneToMany to the ProjectAssociation. Some JPA providers also provide additional support for mapping to join tables with additional data.

Unfortunately mapping this type of model becomes more complicated in JPA because it requires a composite primary key. The association object’s Id is composed of the Employee and Project ids. The JPA spec does not allow an Id to be used on a ManyToOne so the association class must have two duplicate attributes to also store the ids, and use an IdClass, these duplicate attributes must be kept in synch with the ManyToOne attributes. Some JPA providers may allow a ManyToOne to be part of an Id, so this may be simpler with some JPA providers. To make your life simpler, I would recommend adding a generated Id attribute to the association class. This will give the object a simpler Id and not require duplicating the Employee and Project ids.

This same pattern can be used no matter what the additional data in the join table is. Another usage is if you have a Map relationship between two objects, with a third unrelated object or data representing the Map key. The JPA spec requires that the Map key be an attribute of the Map value, so the association object pattern can be used to model the relationship.

If the additional data in the join table is only required on the database and not used in Java, such as auditing information, it may also be possible to use database triggers to automatically set the data.

Example join table association object annotations

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@Entity@Table(name = "product")public class Product {    private Integer id;    private String name;    private List<ProductItem> productItems = new LinkedList<ProductItem>();    public Product() {    }    @Id    @GenericGenerator(name = "generator", strategy = "increment")    @GeneratedValue(generator = "generator")    @Column(name = "product_id", nullable = false)    public Integer getId() {        return this.id;    }    public void setId(Integer id) {        this.id = id;    }    @Column(name = "name")    public String getName() {        return this.name;    }    public void setName(String name) {        this.name = name;    }    /*     * Here is the annotation to add in order to     * Hibernate to automatically insert and update     * ProducItems (if any)     */    @OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.product", cascade =    {CascadeType.PERSIST, CascadeType.MERGE})    @Cascade({org.hibernate.annotations.CascadeType.SAVE_UPDATE,    org.hibernate.annotations.CascadeType.DELETE_ORPHAN})    public List<ProductItem> getProductItems() {        return this.productItems;    }    public void setProductItems(List<ProductItem> productItems) {        this.productItems = productItems;    }}

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Since I only have experience with Hibernate I invite all the reader that have expertise with other JPA provider to reply to this post with the solution offered by other framework such as iBatis.

Thanks for have reading this tutorial and feel free to reply with comments.

摘自：

http://sieze.wordpress.com/2009/09/04/mapping-a-many-to-many-join-table-with-extra-column-using-jpa/