Hibernate - How to Define Association Mappings between Entities - HowToDoInJava
Association mappings are one of the key features of JPA and Hibernate. They model the That is a typical example for a many-to-one association. If you want to. This article takes a look at Hibernate mapping and also explores the They establish the relationship between two database tables as attributes in your along with a foreign key of address table column (in this example). Hibernate one to many mapping annotation example, Learn to create N relationship between two entities using foreign key and join table techniques.
Address must not be an Entity as it is a value type, but for this example, we will consider Address as a separate Entity. If we want to achieve two-way binding like User object should have Address and vice-versa.
Hibernate one to many mapping annotation example
Then, the address table will also contain the foreign key of the user column. You will see the name of the field in both the table will be default depending on table name but to change the default implementation, we have to annotate it with JoinColumn with an attribute name.
As one user can have more than one Mobile. For two-way binding, we annotate the user inside the Mobile class with ManyToOne. You will see the mapping table and its column name will have a default name generated based on the two tables but to change the default implementation, andwe will annotate it JoinTable and the attribute name in this is to change the name of mapping table and attribute joinColumns for the primary key of the same class User in this case and inverseJoinColumns for the primary key of corresponding class Mobile in this case.
We will let consider a world where there are multiple owner of a Vehicle and A owner can also own multiple Vehicle. You can consider that the Vehicle is used for the rent and one User can rent multiple Vehicle and a Vehicle can be rented by multiple Users.
After running the application if we look after the table created by the hibernate, we will found that the there is two mapping table formed one made by first mapping by User in this case and other mapping table by Vehicle class. 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. 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. 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.
But there is another thing you should do to make it easier to use the bidirectional relationship.
Hibernate One-to-Many Mappings
You need to update both ends of a bidirectional association when you want to add or remove an entity. 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. Similar to the previously discussed bidirectional mappings, the bidirectional one-to-one relationship consists of an owning and a referencing side.
The owning side of the association defines the mapping, and the referencing one just links to that mapping. The definition of the owning side of the mapping is identical to the unidirectional mapping.