JPA 2 Relationship JOIN Named Query - sql

Question

I'm using SpringBoot 2.4 with JPA 2.0 and I have a Model like following:
#Entity
#Data
public class Nation {
#Id
#GeneratedValue (strategy = GenerationType.IDENTITY)
private Integer id;
#OneToMany(mappedBy = "nation")
private List<Country> country;
}
And:
#Entity
#Data
public class Country {
#Id
#GeneratedValue (strategy = GenerationType.IDENTITY)
private Integer id;
#ManyToOne
private Nation nation;
}
Now I would to like to find all Nation filtered by their ID and by country ID. In pure SQL is simply something like:
select * from nation n, country c where n.id = [nation_id] AND c.id = [country_id];
Therefore I thought about doing this way with JPA:
#Query("select n from Nation n JOIN n.country c where n.id = ?1 AND c.id = ?2)
public List<Nation> find(Integer nationID, Integer countryID);
But it doesn't work; it is filtered by Nation but not by countries.
If I print the Hibernate generate SQL by adding:
spring.jpa.show.sql=true
I can see that the query is exactly the same I posted above in pure SQL. The problem occours when I invoke nation.getCountry(), it generates another query that load all country connected to given Nation id.
Is there a way to solve this?

Answer 1

I believe that you can use JPA DTO projections for this case ...
So, in your Nation class create a constructor like this:
/**
* Copy Constructor. It creates a "detached" copy of the
* given nation with only a copy of the provided country.
*/
public Nation(Nation n, Country c) {
super();
this.id = n.id;
// copy other nation values ...
this.country.add( new Country(c) );
}
Modify your query to invoke such constructor ... something like this (assuming that Nation is declared in the java package my.domain):
#Query("select new my.domain.Nation(n, c) from Nation n JOIN n.country c where n.id = ?1 AND c.id = ?2)
Disclaimer:
I have done this using JPA and hibernate. So far, I haven't tested with spring, but I guess this does not matter because your JPA provider is probably hibernate too.
I have done this using only parts (or attributes) of the target entities (as described in the provided link) ... I never have pass the full entities (as I suggest to do in the query). Let me explain, in the cases where I have applied this, I have a constructor like Nation(String name, int population) and in the query I do something like: SELECT new my.domain.Nation(n.name, c.population) ... Try to pass the full entities to see if it works ... if it fails, fallback to create a constructor that receives only the attributes that you require for your business case.