JPA:连接表语法

Question

给定以下实体(为简洁起见,此长定义中省略了一些列):

@Table(name = "Products")
public class Products implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @Basic(optional = false)
    @Column(name = "SKU")
    private String sku;
    @Basic(optional = false)
    @Column(name = "ProductName")
    private String productName;

    private boolean allowPreOrder;
    @ManyToMany(mappedBy = "productsCollection")
    private Collection<Categories> categoriesCollection;
    @JoinTable(name = "Products_CrossSell", joinColumns = {
        @JoinColumn(name = "SKU", referencedColumnName = "SKU")}, inverseJoinColumns = {
        @JoinColumn(name = "CrossSKU", referencedColumnName = "SKU")})
    @ManyToMany
    private Collection<Products> productsCollection;
    @ManyToMany(mappedBy = "productsCollection")
    private Collection<Products> productsCollection1;
    @JoinTable(name = "Products_Related", joinColumns = {
        @JoinColumn(name = "SKU", referencedColumnName = "SKU")}, inverseJoinColumns = {
        @JoinColumn(name = "RelatedSKU", referencedColumnName = "SKU")})
    @ManyToMany
    private Collection<Products> productsCollection2;
    @ManyToMany(mappedBy = "productsCollection2")
    private Collection<Products> productsCollection3;

如何获得给定产品SKU的相关产品集？

products_related表如下所示:

我知道如何使用SQL获得答案,但我是JPA的新手,所以我还没有完全了解API和查询语法.

Answer 1

在我看来，定义了一些不必要的集合。反正：

@JoinTable(name = "Products_Related", joinColumns = {
    @JoinColumn(name = "SKU", referencedColumnName = "SKU")}, inverseJoinColumns = {
    @JoinColumn(name = "RelatedSKU", referencedColumnName = "SKU")})
@ManyToMany
private Collection<Products> productsCollection2;

这一部分（它存在于您的代码中）应该为您提供所需的产品。只需将其重命名为relatedProducts, 以及相应的 setter/getter 即可。

更新：您可以通过以下方式获取对象：

Product p = entityManager.find(Product.class, yourProductId);
p.getRelatedProducts();

获取实体管理器取决于您的设置，并且寻找如何获取它的更好地方是教程。