java hibernate entity: allow to set related object both by id and object itself

Question

I've got a following Java class which is also a Hibernate entity:

@Entity
@Table(name = "category")
public class Category {

    @ManyToOne
    @JoinColumn(name="parent_id")
    private Category parent;

    public Category getParent() {
        return parent;
    }

    public void setParent(Category parent) {
        this.parent = parent;
    }

The category represents a node in a category tree. I'm implementing a webservice which allows to CRUD categories. For instance, the interface has the ability to create a category tree node and it passes the category id as a parameter.

I want just to create a new Category object and persist it into the database without fetching the parent object. My data provider class looks like this:

public void createCategory(int parent_id, String name, CategoryType type) {
    Category category = new Category();
    category.setName(name);
    // category.setParent(?); <- I don't have this object here
    // category.setParentId(id); <- but I do have the id
    category.setType(type);
    this.categoryDao.save(category);
}

My question is: what can I do to create a new Category object with the parent_id set if assuming I won't call hibernate to fetch the parent for me (this would be stupid)? Can I provide a setParentId/getParentId method for Category class? What hibernate annotations would it have?

@MagnusTengdahl I've got no idea how to do the annotation mapping... — ducin
– ducin, Commented Apr 16, 2013 at 18:04

axtavt · Accepted Answer · 2013-04-16 18:09:02Z

22

Hibernate provides a method called (quite confusingly) Session.load() for this scenario.

Session.load() returns a lazy proxy with the given identifier without querying the database (if object with the given identifier is already loaded in the current Session, it returns an object itself).

You can use that proxy to initialize relationships in your entities being saved:

category.setParent(session.load(Category.class, parent_id));

Note that this code doesn't check existence of Category with the given id. However, if you have a foreign key constraint in your DB schema, you'll get a constraint violation error when invalid id is passed in.

JPA equivalent of this method is called EntityManager.getReference().

answered Apr 16, 2013 at 18:09

axtavt

243k42 gold badges518 silver badges486 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

ducin Over a year ago

I've got one problem here: my architectural design assumes I'm using BO/DAO (Business object / Data access object). DAO is the hibernate level and it holds the hibernate session. BO (the one in my second code snippet) doesn't even know that a thing such as hibernate session exists. But - according to your code - I have to use the session.load. What should I change in my architecture?

ducin Over a year ago

If I want my Category BO class to be generic, maybe I should provide setParentId method for the category, call it in the BO and process it on the DAO level - what do you think about that?

axtavt Over a year ago

I think that the best option is to add a corresponding method to your DAO, something like Category toReference(long categoryId).

ducin Over a year ago

so it would be - more or less - "DAO, please give me a lazy-reference to a parent category object which you will use later on (I need it only just for now). I don't know how you do it, but I guess you know that". Right?

axtavt Over a year ago

Yes. It would make your DAO abstraction somehow leaky, but it's inevitable in this case.

|

Ondrej Burkert · Accepted Answer · 2014-01-14 12:38:45Z

There is one more solution to problem - using the default constructor of the entity you want a reference for and set id and version (if it is versioned).

        Constructor<S> c = entityClass.getDeclaredConstructor();
        c.setAccessible(true);

        S instance = c.newInstance();
        Fields.set(instance, "id", entityId.getId());
        Fields.set(instance, "version", entityId.getVersion());
        return instance;

We can use such an approach because we don't use lazy loading but instead have 'views' of entities which are used on the GUI. That allows us to get away from all the joins Hibernate uses to fill all eager relations. The views always have id and version of the entity. Hence, we can fill the reference by creating an object which would appear to Hibernate as not transient.

I tried both this approach and the one with session.load(). They both worked fine. I see some advantage in my approach as Hibernate won't leak with its proxies elsewhere in the code. If not properly used, I'll just get the NPE instead of the 'no session bound to thread' exception.

StanislavL · Accepted Answer · 2013-04-16 18:11:40Z

-2

You can define lazy init

@ManyToOne(fetch=FetchType.LAZY)

guess the column is nullable so it's not a problem to save the Category without parent (root)

answered Apr 16, 2013 at 18:11

StanislavL

57.5k9 gold badges75 silver badges100 bronze badges

1 Comment

JB Nizet Over a year ago

That doesn't answer the question. And the goal is precisely to create a category with a parent.

Collectives™ on Stack Overflow

java hibernate entity: allow to set related object both by id and object itself

3 Answers 3

6 Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

6 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related