java - One-To-Many Relationship in Spring Data JPA - Stack Overflow
With JPA and Hibernate, you can model this in 3 different ways. You can The unidirectional one-to-many relationship mapping is not very common. In the. This post walks you through the process of mapping an one-to-many relationship with Spring Boot, Spring Data JPA and MySQL. JPA Series. This kind of association is mapped by JPA using the @ManyToOne Creating the @OneToMany and @ManyToOne relationship.
This element is also of particular importance on the OneToMany annotation because it will allow persistence of data from the inverse side of the relationship. JoinColumn The JoinColumnannotation is used to specify a column on the database table for the relationship. So, in our example, in the EmployeeEntity bean we got the following: In this case, the name of the FK column is "company" as defined in the constraint. The referencedColumnName element of the JoinColumn annotation is used to define the column in the referenced table for the relationship, which is our case is "idcompany", which is the Primary Key PK of the referenced table.
There are many other element options available for this annotation and one particularity about them is that they are all concerning the name element of the JoinColumn. Because the relationship we are creating is bidirectional because references exist on both sides of the relationship the annotation element mappedBy MUST be used to specify the field that exists on the referenced entity which is the owner of the relationship.
The other possible annotation elements that can be used are cascade, fetch and targetEntity, which work exactly as explained for the ManyToOne annotation and the element orphanRemoval which is defaulted to false.
The orphanRemoval functionality is intended for entities that are privately owned by the parent entity and will cause a remove operation to be propagated to those child entities without the need to use the cascade type remove to make it so. Bear in mind that having this side of the relationship with the element cascade as ALL has several implications with the working of the code itself and the management of the entity beans. This will be shown in a bit.
And that is it! Obviously this wouldn't be complete without some testing and demonstration of the topics just discussed here! So lets jump right into that: Seeing it working So for starters, all the code for this project can be found at: Also, there should be a database configured on your test box, the Schema MySQL Workbench file can be found inside the distribution in a folder called model.
Add the following property line there: You could configure the database to log all SQL queries if you would like, I rather do it this way cause I'll also get verbose output from the Entity Manager that actually help solve loads of problems. Also, adjust the remaining properties of the persistence. Now go for the Test Packages and open the only JUnit file there: This file has two tests configured in it but I'm not actually using any asserts there to validate the correctness of the Tests, but instead just looking to see if the operations return exceptions a Fail or if all operations on the method execute without exception.
OK lets do it then, run the JUnit tests. So all went well hopefully. Lets go ahead and delete testCreateCompany test method or just comment the Test annotation and run it again. The output of the log should show something like this I trimmed a bit of the beginning of each line of the log: There is another entity bean on the code, an EmployeeEntity object created but never persisted. The reason why in the first bit of logging, corresponding to the em.
JPA 2 Tutorial - Relationships - One To Many
Is it necessary for both the Entity Objects to reference each other, could we not just make one reference the other and be done with it? JPA 2 Spec states in page 42 the following: Basically, the application should maintain consistency of run-time relationships, not the Entity Manager, and there is good reason for that: If only one side of the bidirectional relationship has a reference to the other side of the relationship then at run-time both Entities tell a different story about their relationship and More on this later.
If for example we were to comment the following line: Column 'company' cannot be null Error Code: 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.#12 Hibernate Tutorial - Mapping Relations Theory
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.
You need to update both ends of a bidirectional association when you want to add or remove an entity.
- JPA - Entity Relationships
- Ultimate Guide – Association Mappings with JPA and Hibernate
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.
JPA Entity Relationships
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.
The bidirectional associations consist of an owning and a referencing side. The owning side of the association is identical to the unidirectional mapping and defines the mapping. The referencing side only links to the attribute that owns the association.