java - HQL多对多查询

Question

我有两个具有多对多关系的表。 products(id, description, price,image)----> products_category(idProducts, category _id)----> category(id, category_name).

这是我的实体:

<强>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.类别

@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 Controller 编写查询，它会通过先前选择的带有 id 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

如果您需要检索产品，您需要执行选择产品实体而非类别的查询。

所以:

return all the products by previosly selected category object with id

你需要做的:

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 但这在您的情况下不是必需的，因为您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，始终强制执行 JOIN。