Tag: many-to-many
Nabin Nepal (Starx) java jpa many-to-many named-query
June 3, 2012

How to create a namedquery of manytomany entity?

Question by Khushbu Joshi

Khushbu JoshiKhushbu Joshi

Brand

public class Brand implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "BrandID", nullable = false)
    private Integer brandID;
    @Basic(optional = false)
    @Column(name = "BrandName", nullable = false, length = 100)
    private String brandName;
    @Basic(optional = false)
    @Column(name = "Description", nullable = false, length = 1000)
    private String description;
    @Column(name = "Is_Visible")
    private Boolean isVisible;
    @JoinTable(name = "brandcategory", joinColumns = {
        @JoinColumn(name = "BrandID", referencedColumnName = "BrandID")}, inverseJoinColumns = {
        @JoinColumn(name = "CategoryID", referencedColumnName = "CategoryID")})
    @ManyToMany(fetch = FetchType.EAGER)
    private Collection<Category> categoryCollection;
    @OneToMany(mappedBy = "brand", fetch = FetchType.EAGER)
    private Collection<Product> productCollection;

I want to retrive the Brand IDs from table brandcategory whoes categoryID = :categoryID
how can i createnamed query for it in entity brand?

this does not work:

@NamedQuery(name = "Brand.getBrandListByCategory",
            query = "SELECT b FROM Brand b WHERE b.brandID =
            (SELECT bc.brandID
             FROM b.brandctegory bc
             WHERE bc.category.categoryID = :categoryID)")

Answer by JB Nizet

JB NizetJB Nizet

If I understand correctly, you want all the brands belonging to a category. Why don’t you simply make the association bidirectional. You could then just do:

Category category = em.find(Category.class, categoryId);
return category.getBrands();

If it’s unidirectional, then you’ll need a query, but it’s much simpler that the one you tried:

select b from Brand b inner join b.categoryCollection category 
where category.id = :categoryId;

Your query doesn’t make sense: it uses a non-existing association (b.brandcategory). Remember that JPQL uses entities, their persistent fields and associations to other entities. And nothing else. Tables don’t exist in JPQL.

Answer by Starx

StarxStarx

AFAIK, you cant go out of a entity boundary, when creating queries in entity class.

Instead use .createNativeQuery() method of the entity manager, to create complex and mixed queries.

Nabin Nepal (Starx) class many-to-many oop php
March 31, 2012

In Object Oriented Programming which object should maintain the many to many relationship? (if there is one)

Question by Matt

MattMatt

I’ll use an example to illustrate this:

class Company {

}

class Person {

}

Company and Person hold a many to many relationship. A Person can belong to multiple Companies and a Company can hold multiple People.

Would I then need to create a third class:

class CompanyPerson {

}

or should the company handle it:

class Company {
    function add_person() {

    }
}

or maybe the Person should?

class Person {
    function add_to_company() {

    }
}

Answer by Starx

StarxStarx

This depends on the use case, but generally many to many object relationship can be deployed using a reference class

class CompanyPersonRelationship {
    public $company;
    public $person;
}

So now, both company and person can keeps track of their relationship lie

class Company {
   public $persons = array();   
}

class Person {
   public $companies = array();
}
...

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