Hibernate one to relationship

java - Update using one to one relationship hibernate - Stack Overflow

hibernate one to relationship

5 days ago We have seen how easy it is to implement the one-to-many relationship with the Hibernate ORM and MySQL database using JPA annotations. Association mappings are one of the key features of JPA and Hibernate. They model the relationship between two database tables as attributes in your domain . Since you have already mapped Address entity in employee entity like this @ OneToOne(mappedBy="employee", cascade = CascadeType.

hibernate one to relationship

You need to update both sides of the association. You should also take a look at FetchTypes and how they impact the way Hibernate loads entities from the database.

hibernate one to relationship

Many-to-Many Associations Many-to-Many relationships are another often used association type. On the database level, it requires an additional association table which contains the primary key pairs of the associated entities. A typical example for such a many-to-many association are Products and Stores.

Each Store sells multiple Products and each Product gets sold in multiple Stores.

Hibernate – One-to-One example (Annotation)

Similar to the many-to-one association, you can model a many-to-many relationship as a uni- or bidirectional relationship between two entities. But there is an important difference that might not be obvious when you look at the following code snippets. When you map a many-to-many association, you should use a Set instead of a List as the attribute type. Otherwise, Hibernate will take a very inefficient approach to remove entities from the association.

It will remove all records from the association table and re-insert the remaining ones. You can avoid that by using a Set instead of a List as the attribute type.

Unidirectional Many-to-Many Associations Similar to the previously discussed mappings, the unidirectional many-to-many relationship mapping requires an entity attribute and a ManyToMany annotation. The attribute models the association and you can use it to navigate it in your domain model or JPQL queries.

The annotation tells Hibernate to map a many-to-many association. The Set products attribute models the association in the domain model and the ManyToMany association tells Hibernate to map it as a many-to-many association.

And as I already explained, please note the difference to the previous many-to-one mappings. You should map the associated entities to a Set instead of a List.

Best Practices for Many-To-One and One-To-Many Association Mappings

You can customize that with a JoinTable annotation and its attributes joinColumns and inverseJoinColumns. The joinColumns attribute defines the foreign key columns for the entity on which you define the association mapping.

The inverseJoinColumns attribute specifies the foreign key columns of the associated entity. You can now use it to get a Set of associated entities in your domain model or to join the mapped tables in a JPQL query. One of the two entities owns the association and provides all mapping information. The other entity just references the association mapping so that Hibernate knows where it can get the required information. The mapping is identical to the unidirectional many-to-many association mapping.

You need an attribute that maps the association in your domain model and a ManyToMany association. If you want to adapt the default mapping, you can do that with a JoinColumn annotation. Similar to the bidirectional many-to-one relationship mappingyou just need to reference the attribute that owns the association. You can see an example of such a mapping in the following code snippet. The List products attribute of the Store entity owns the association.

hibernate one to relationship

But there is another thing you should do to make it easier to use the bidirectional relationship. You need to update both ends of a bidirectional association when you want to add or remove an entity.

hibernate one to relationship

Doing that in your business code is verbose and error-prone. One-to-One Associations One-to-one relationships are rarely used in relational table models. But you will run into it from time to time. An example for a one-to-one association could be a Customer and the ShippingAddress. On the database level, this mapped by a foreign key column either on the ShippingAddress or the Customer table. Unidirectional One-to-One Associations As in the previous unidirectional mapping, you only need to model it for the entity for which you want to navigate the relationship in your query or domain model.

The required mapping is similar to the previously discussed mappings. You need an entity attribute that represents the association, and you have to annotate it with an OneToOne annotation.

When you do that, Hibernate uses the name of the associated entity and the name of its primary key attribute to generate the name of the foreign key column. You can customize the name of the foreign key column with a JoinColumn annotation. You can now use it in your business to add or remove an association, to navigate it in your domain model or to join it in a JPQL query. In this example, you also model it on the ShippingAddress entity so that you can get the Customer for a giving ShippingAddress.

But you should avoid unidirectional one-to-many associations in your domain model. Otherwise, Hibernate might create unexpected tables and execute more SQL statements than you expected. You just need an attribute that maps the association and a OneToMany relationship. You probably expected that Hibernate would only persist a new Item entity in the item table.

I did the same when I used this mapping for the first time. Why does Hibernate execute so many queries and introduce an additional association table? In your table model, you normally use a foreign key column on the to-many side of the association to store a reference to the associated record. Hibernate uses the same approach when you model a bidirectional one-to-many or an unidirectional many-to-one relationship.

It uses the foreign key column to map the association. So, Hibernate introduces an association table to store the foreign keys. You can avoid this table if you specify the foreign key column with a JoinColumn annotation.

This column has to be part of the table of the to-many side of the association. So, better use a bi-directional instead of a unidirectional one-to-many association. Avoid the mapping of huge to-many associations I know, mapped to-many associations are useful, especially when you want to join entities in a JPQL query.

But Hibernate loads all associated entities when it initializes the association. That can take several seconds or even minutes when Hibernate has to fetch several thousand entities.

Hibernate - One-to-One Mappings

So, better use an unidirectional many-to-one association. That allows you to fetch a number of entities that you can handle in your business logic or present to the user. Think twice before using CascadeType. Remove Cascade remove is another feature that works well on small to-many associations. Using it for one-to-many or many-to-one associations is not as dangerous as it is for many-to-many relationships. The following mapping tells Hibernate to remove all associated Item entities when it deletes the PurchaseOrder entity.