Retrieve data from three entities where two of them are in many-to-one relationships with the last one - json

Question

I am using Spring Boot as REST backend app. Let's say that Person has Cats and Dogs and both Cat and Dog are in many-to-one relathionships with Person. By this, Cat and Dog have Person id as a foreign key. Since I'm using Spring JPA Repository and many-to-one relationship, it is straightforward to get list of cats with their persons and list of dogs with their persons. Those lists are transformed to json and I can access the person's data with frontend app. Here is my problem:
I want to return the list of all persons with all the cats and dogs for each person.
I guess that JPA Repository does not have a default query for my request, so I have to use custom queries. However, I do not know how to make it. I have tried the following one:
#Query("select p, c, d from Person p, Cat c, Dog d where c.person.id = :id and d.person.id = :id and person.id = :id")
List<Object[]> findAllPersonsWithCatsAndDogs(Integer id);
The idea was to run through for loop for each person and to use person's id to retrieve his cats and dogs. The result is the list of objects where each object has the same person, one of his cats, and one of his dogs. I do not like that, because then for all persons I have a list of lists of persons with their cats and dogs.
How to get one list of all persons with all the cats and dogs for each person.
Thanks
Here are the mappings to make it more clear:
#Entity
public class Person {
//there is no mappings because of unidirectional many to one
}
...
#Entity
public class Cat{
#ManyToOne
#JoinColumn(name = "person_id")
private Person person;
}
...
#Entity
public class Dog{
#ManyToOne
#JoinColumn(name = "person_id")
private Person person;
}
So, I have many to one unidirectional, which means that Person does not see cats and dogs.

Answer 1

You should add the reference between person, cat and dog.
#Entity
public class Person {
#OneToMany(cascade = CascadeType.PERSIST, mappedBy="person")
private List<Cat> catList;
#OneToMany(cascade = CascadeType.PERSIST, mappedBy="person")
private List<Dog> dogList;
}
Then if you want to get all the people with their cats and dogs you can do something like
#Query("SELECT p FROM Person P JOIN FETCH p.catList JOIN FETCH p.dogList")
List<Person> findAllPersonsWithCatsAndDogs()
(catList and dogList are just assumed to be the list in person due to not seeing your mapping).
This query will eager fetch your cats and dog list for every person. Then you can do
for (Person p : personRepo.findAllPersonsWithCatsAndDogs()) {
for (Cat c : p.getCastList()) {
}
for (Dog d : p.getDogList()) {
}
}

Answer 2

In general, the DTO Pattern is used to wrap up the necessary data. Therefore, you can use several object mapping frameworks such as ModelMapper, Modelstruct, Dozer.
If you create two repositories for the cat and dog Entity, you can do the following approach:
Method in CatRepository:
List<Cat> findByPersonId(int id);
Method in DogRepository:
List<Cat> findByPersonId(int id);
Querying the persons:
List<Person> persons = personRepository.findAll();
List<PersonDto> personsDto = new ArrayList<>();
foreach(Person p:persons) {
PersonDto dto = modelmapper.map(p, PersonDto.class);
p.setCats(catRepo.findByPersonId(p.getId()));
p.setCats(catRepo.findByPersonId(p.getId()));
personsDto.add(dto);
}