通过JPA联接从父实体返回子实体

Question

How can I return a list of entities that has a relationship to a parent in JPA?

I have a User entity that has a @OneToMany mapping on a property named pets . The child entities are of type Pet. It is only a uni-directional relationship.

How can I write a join in JPA that returns all pets given a user?

Answer 1

So you have a couple of options.

You can use the following annotations:

@ManyToOne
@JoinColumn

This is how you would use it.

public class User
{
   // your standard fields / columns in database

   @OneToMany  (Fetch can be of eager/ lazy)
   @JoinColumn (name="column to join on", referencedColumnName="column to join on in parent class")
   private List<Pet> pets;
}


public Class Pet
{
   //data fields
}

What essentially happens is the list of pets is populated when you are querying for the user object.

Using JPA to Query the DB.

So i am guessing that Your user would have some sort of id and the pet table would have some sort of Id to the user that are linked.

So we would do the following

Select * from user  where user_id = ?;

this will essentially give you the user object

Select * from pet where owner_user_id = ?

this will essentially give you all the pets that belong to that user.

Then you can populate your object yourself.

I am not 100% sure of how your table looks like, but I was hoping to give it a stab from just what I would do point of view.