HQL多对多查询

Question

我有两张桌子，有很多关系。 产品（id，description，price，image）----> products_category（idProducts，category _id）----> category（id，category_name）。

这是我的用法：

1.产品

@Entity
@Table(name = "products")
public class Products implements Serializable {

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

    @Column(name = "description")
    private String description;

    @Column(name = "price")
    private String price;

    @Column(name = "image")
    private byte [] image;

    public Products() {
    }

    public Products(String description, String price, byte[] image) {}


    @ManyToMany
    @JoinTable(
       name = "category_products",
            joinColumns ={@JoinColumn (name = "Products_idProducts", referencedColumnName = "idProducts")},
            inverseJoinColumns = {@JoinColumn(name = "category_id", referencedColumnName = "id")}
    )
        List<Category> categories = new ArrayList<>();

    @ManyToMany
    @JoinTable(
            name = "users_product",
            joinColumns ={@JoinColumn (name = "Products_idProducts", referencedColumnName = "idProducts")},
            inverseJoinColumns = {@JoinColumn(name = "users_id", referencedColumnName = "id")}
    )
      List<Users> usersList = new ArrayList<>();

2.Category

@Entity
@Table(name = "category")
public class Category {
    @Id
    @Column(name = "id")
    private long id;

    public Category() {
    }

    public Category(String category_name) {
        this.category_name = category_name;
    }

    @Column (name = "category_name")
    private String category_name;

    @ManyToMany(mappedBy = "categories")
    private List<Products> products = new ArrayList<>();

我正在尝试为控制器编写查询，它通过带有id的先前选择的类别对象返回所有产品？ 我尝试了很多查询，但都抛出异常。

public List<Products> list (Category category) {
         //category - object with needed id
        Query query;
            query = entityManager.createQuery("SELECT c FROM Category c  left join c.categories WHERE c.category = :category", Products.class);
            query.setParameter("category", category);

        return (List<Products>) query.getResultList();
    }

java.lang.IllegalArgumentException: org.hibernate.QueryException: could not resolve property: categories of: classes.Category [SELECT c FROM classes.Category c join c.categories WHERE c.category = :category]
    org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1750)
    org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1677)
    org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1683)

Answer 1

如果需要检索产品，则需要执行select产品实体”而不是“类别”的查询。

所以：

通过具有id的先前选择的类别对象返回所有产品

你需要这样做：

Query query = entityManager.createQuery("SELECT p FROM Product p 
    JOIN p.categories c 
    WHERE c.id = :idCategory");
query.setParameter("idCategory", category.getId());

您使用LEFT JOIN但在您的情况下这不是必需的，因为查询的唯一条件是找到具有特定ID的类别。 此条件将忽略JOIN的LEFT部分，强制始终为JOIN 。

Answer 2

我认为您的代码中存在一些错误：

1. 您SELECT c表示Category但是您将结果列表转换为List<Product> ，应该是SELECT c.products

2. 您的WHERE c.category = :category子句不正确，因为您的Category类中没有任何category属性，应该是WHERE c.id = :id和query.setParameter("id", category.getId());

希望有所帮助。