Ultimate Guide - Association Mappings with JPA and Hibernate
Example 1: One-to-one association that maps a foreign key column // On Customer class: @OneToOne(optional=false) @JoinColumn(name=" CUSTREC_ID". To my knowledge, Hibernate doesn't support optional one-to-one (see HHH- ) so you'll have to use a fake many-to-one with not-null="false" instead. In this week's Hibernate Tip, I show you how to map a to-one association to an Optional.
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.
JPA - Making @OneToOne relationship mandatory by setting optional=false
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. 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.
The best way to map a @OneToOne relationship with JPA and Hibernate
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.
Learning the code way: One to one association that is optional
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. It consists of an attribute that models the relationship and is annotated with a OneToOne annotation and an optional JoinColumn annotation.
You can define that with the mappedBy attribute of the OneToOne annotation. Summary The relational table model uses many-to-many, many-to-one and one-to-one associations to model the relationship between database records. You can map the same relationships with JPA and Hibernate, and you can do that in an unidirectional or bidirectional way. The unidirectional mapping defines the relationship only on 1 of the 2 associated entities, and you can only navigate it in that direction.
The bidirectional mapping models the relationship for both entities so that you can navigate it in both directions. The concept for the mapping of all 3 kinds of relationships is the same. Hibernate will generate the name of the required foreign key columns and tables based on the name of the entities and their primary key attributes.
Here the unique attribute is not set. Optional attribute and not-null attribute is set to "true" to indicate that the relation is not always there. The inverse attribute is set here to ensure that the relation is not manged from here.
Cascade settings have also been included. On start up the hibernate logs indicate the generated SQL: IdentifierGeneratorFactory - Natively genera ted identity: SessionImpl - after transaction completion The Chocolate Bag with name Bag01 was created with id 1 Chocolate1 saved with id 1 To create the relationship between the two entities I execute the below code: IntegerType - binding '1' to parameter: In the first session, the code to load the chocolate resulted in a join query that will return at most a single chocolate record.
Attempt to access the bag via the association results in an additional select query. For the second session the queries are as below: The attempt to load the Bag object will result will result in a simple select like above.
Loading the one to many association will result in the second double join query: To delete a bag, the code below was used: First the records were removed from the link table, then the chocolates associated and then the bag record.
- Hibernate One to One Mapping Example(Annotation Based)
- Ultimate Guide – Association Mappings with JPA and Hibernate
- Hibernate Tips: How to map an association to an Optional