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1. Introduction

This quick Hibernate tutorial will take us through an example of a one-to-many mapping using JPA annotations, an alternative to XML.

We'll also learn what bidirectional relationships are, how they can create inconsistencies, and how the idea of ownership can help.

Further reading:

Spring Boot with Hibernate

A quick, practical intro to integrating Spring Boot and Hibernate/JPA.

An Overview of Identifiers in Hibernate/JPA

Learn how to map entity identifiers with Hibernate.

2. Description

Simply put, one-to-many mapping means that one row in a table is mapped to multiple rows in another table.

Let’s look at the following entity-relationship diagram to see a one-to-many association:

C-1

For this example, we'll implement a cart system where we have a table for each cart and another table for each item. One cart can have many items, so here we have a one-to-many mapping.

The way this works at the database level is we have a cart_id as a primary key in the cart table and also a cart_id as a foreign key in items.

The way we do it in code is with @OneToMany.

Let's map the Cart class to the collection of Item objects in a way that reflects the relationship in the database:

public class Cart {

    //...     
 
    @OneToMany(mappedBy="cart")
    private Set<Item> items;
	
    //...
}

We can also add a reference to Cart in each Item using @ManyToOne, making this a bidirectional relationship. Bidirectional means that we are able to access items from carts, and also carts from items.

The mappedBy property is what we use to tell Hibernate which variable we are using to represent the parent class in our child class.

The following technologies and libraries are used in order to develop a sample Hibernate application that implements one-to-many association:

  • JDK 1.8 or later
  • Hibernate 5
  • Maven 3 or later
  • H2 database

3. Setup

3.1. Database Setup

We'll use Hibernate to manage our schema from the domain model. In other words, we don't need to provide the SQL statements to create the various tables and relationships between the entities. So let's move on to creating the Hibernate example project.

3.2. Maven Dependencies

Let's start by adding the Hibernate and H2 driver dependencies to our pom.xml file. The Hibernate dependency uses JBoss logging, and it automatically gets added as transitive dependencies:

  • Hibernate version 5.6.7.Final
  • H2 driver version 2.1.212

Please visit the Maven central repository for the latest versions of Hibernate and the H2 dependencies.

3.3. Hibernate SessionFactory

Next, let's create the Hibernate SessionFactory for our database interactions:

public static SessionFactory getSessionFactory() {

    ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder()
      .applySettings(dbSettings())
      .build();

    Metadata metadata = new MetadataSources(serviceRegistry)
      .addAnnotatedClass(Cart.class)
      // other domain classes
      .buildMetadata();

    return metadata.buildSessionFactory();
}

private static Map<String, String> dbSettings() {
    // return Hibernate settings
}

4. The Models

The mapping-related configurations will be done using JPA annotations in the model classes:

@Entity
@Table(name="CART")
public class Cart {

    //...

    @OneToMany(mappedBy="cart")
    private Set<Item> items;
	
    // getters and setters
}

Please note that the @OneToMany annotation is used to define the property in Item class that will be used to map the mappedBy variable. That is why we have a property named “cart” in the Item class:

@Entity
@Table(name="ITEMS")
public class Item {
    
    //...
    @ManyToOne
    @JoinColumn(name="cart_id", nullable=false)
    private Cart cart;

    public Item() {}
    
    // getters and setters
}

It's also important to note that the @ManyToOne annotation is associated with the Cart class variable. @JoinColumn annotation references the mapped column.

5. In Action

In the test program, we are creating a class with a main() method for getting the Hibernate Session, and saving the model objects into the database implementing the one-to-many association:

sessionFactory = HibernateAnnotationUtil.getSessionFactory();
session = sessionFactory.getCurrentSession();
System.out.println("Session created");
	    
tx = session.beginTransaction();

session.save(cart);
session.save(item1);
session.save(item2);
	    
tx.commit();
System.out.println("Cart ID=" + cart.getId());
System.out.println("item1 ID=" + item1.getId()
  + ", Foreign Key Cart ID=" + item.getCart().getId());
System.out.println("item2 ID=" + item2.getId()
+ ", Foreign Key Cart ID=" + item.getCart().getId());

This is the output of our test program:

Session created
Hibernate: insert into CART values ()
Hibernate: insert into ITEMS (cart_id)
  values (?)
Hibernate: insert into ITEMS (cart_id)
  values (?)
Cart ID=7
item1 ID=11, Foreign Key Cart ID=7
item2 ID=12, Foreign Key Cart ID=7
Closing SessionFactory

6. The @ManyToOne Annotation

As we have seen in section 2, we can specify a many-to-one relationship by using the @ManyToOne annotation. A many-to-one mapping means that many instances of this entity are mapped to one instance of another entity – many items in one cart.

The @ManyToOne annotation lets us create bidirectional relationships too. We'll cover this in detail in the next few subsections.

6.1. Inconsistencies and Ownership

Now, if Cart referenced Item, but Item didn't in turn reference Cart, our relationship would be unidirectional. The objects would also have a natural consistency.

In our case though, the relationship is bidirectional, bringing in the possibility of inconsistency.

Let's imagine a situation where a developer wants to add an item1 to the cart1 instance and an item2 to the cart2 instance, but makes a mistake so that the references between cart2 and item2 become inconsistent:

Cart cart1 = new Cart();
Cart cart2 = new Cart();

Item item1 = new Item(cart1);
Item item2 = new Item(cart2); 
Set<Item> itemsSet = new HashSet<Item>();
itemsSet.add(item1);
itemsSet.add(item2); 
cart1.setItems(itemsSet); // wrong!

As shown above, item2 references cart2, whereas cart2 doesn't reference item2, and that's bad.

How should Hibernate save item2 to the database? Will item2 foreign key reference cart1 or cart2?

We resolve this ambiguity using the idea of an owning side of the relationship; references belonging to the owning side take precedence and are saved to the database.

6.2. Item as the Owning Side

As stated in the JPA specification under section 2.9, it's a good practice to mark the many-to-one side as the owning side.

In other words, Item would be the owning side and Cart the inverse side, which is exactly what we did earlier.

So how did we achieve this?

By including the mappedBy attribute in the Cart class, we mark it as the inverse side.

At the same time, we also annotate the Item.cart field with @ManyToOne, making Item the owning side.

Going back to our “inconsistency” example, now Hibernate knows that item2‘s reference is more important and will save item2‘s reference to the database.

Let's check the result:

item1 ID=1, Foreign Key Cart ID=1
item2 ID=2, Foreign Key Cart ID=2

Although cart references item2 in our snippet, item2‘s reference to cart2 is saved in the database.

6.3. Cart as the Owning Side

It's also possible to mark the one-to-many side as the owning side, and the many-to-one side as the inverse side.

Although this is not a recommended practice, let's go ahead and give it a try.

The code snippet below shows the implementation of the one-to-many side as the owning side:

public class ItemOIO {
    
    //  ...
    @ManyToOne
    @JoinColumn(name = "cart_id", insertable = false, updatable = false)
    private CartOIO cart;
    //..
}

public class CartOIO {
    
    //..  
    @OneToMany
    @JoinColumn(name = "cart_id") // we need to duplicate the physical information
    private Set<ItemOIO> items;
    //..
}

Notice how we removed the mappedBy element and set the many-to-one @JoinColumn as insertable and updatable to false.

If we run the same code, the result will be the opposite:

item1 ID=1, Foreign Key Cart ID=1
item2 ID=2, Foreign Key Cart ID=1

As shown above, now item2 belongs to the cart.

7. Conclusion

We have seen how easy it is to implement the one-to-many relationship with the Hibernate ORM and H2 database using JPA annotations.

Additionally, we learned about bidirectional relationships and how to implement the notion of an owning side.

The source code in this article can be found over on GitHub.

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