JPA : How to deal with one to many association

After reading and experiencing lot of solutions for modeling a one to many association, I'll show you the ways you can REALLY do it:

1. Using bidirectional association

the parent entity :


As you can see, I use the @oneToMany annotation with the following properties :
  • mappedBy --> parent property of the child
  • cascade --> ALL by default
  • orphanRemoval --> true by default
  • fetch --> we use LAZY for performance reason
I use a Set for the collection. I provide also a helper method for adding a child to the parent (You can do the same for removing a child).

The child entity below implements also the equals and hashCode methods:



In order to retrieve the parent and the children with ONE query, we have 2 solutions:
  1. @NamedEntityGraph : this annotation tell JPA which node to load automatically. 
  2. @NamedQuery using LEFT JOIN FETCH
These methods are both used in the below repository:

2. @ManyToOne ONLY in the child entity 

The only think you need to do is to create a JPQL query for retrieving the child entities associated with the parent.

SELECT m FROM MeasuringUncertainty m WHERE m.device.identification = :identification

This solution is far better for me because I can also paginate, filter and sort the results.

Happy learning!!

Comments

Post a Comment

Popular posts from this blog

Spring JPA : Using Specification with Projection

You may probably know that using Specification with Projection is not possible at this moment. I'll show you how you can easily bypass this problem using the JPA Criteria. 1. Create an extended repository We begin by creating an extending global repository as follow : You can find more information here . 2. Add our custom method In order to limit the fields returned for each item, the consumer of my API must provide the required fields this way: /orders?fields=productId,name,quantity . ( Best Practices ) Basically, I'm defining the SELECT clause with the returned fields (cf. getSelections() ): 😎Happy coding...
Read more

Chip input using Reactive Form

The Chips component documentation from the angular material website, describes how to use the MatChipInput directive with a chip-list for adding or removing chips. I'll show you how you can implement the same component using Reactive Form. The HTML template: The TS component file:   As you can see, we use the FormBuilder class to simplify the form creation. We define our requirements as a  FormArray within our FormGroup. The requirements will be accessed in our template with this expression inside the *ngFor of <mat-chip></mat-chip> : *ngFor="let requirement of form.get('requirements').controls; let i = index;" In the ChipComponent class, the requirements array is accessed by using this expression: this.form.get('requirements') as FormArray; Thanks
Read more