HQL many to many query

Question

I have two tables with many to many relations. products(id, description, price,image)----> products_category(idProducts, category _id)----> category(id, category_name).

Here is my enteties:

1. Products

@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<>();

I'm try to write query for controller, which return all the products by previosly selected category object with id? i tried many query, but all throws exceptions.

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

If you need to retrieve products, you need to do a query that select Product entity, not Category.

So:

return all the products by previosly selected category object with id

You need to do:

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

You use LEFT JOIN but this is not necessary in your case, because the unique condition of your query is find a category with a specific ID. This condition will ignore the LEFT part of the JOIN , forcing always a JOIN .

Answer 2

I think there are some errors in your code:

1. You SELECT c which mean Category but you cast the result list to List<Product> , should be SELECT c.products

2. Your WHERE c.category = :category clause is not correct because you don't have any category attribute in your Category class, should be WHERE c.id = :id and query.setParameter("id", category.getId());

Hope that help.