How to get a specific element in a collection field of a JPA entity? - spring-data-jpa

Question

I'm using Criteria API to retrieve entities from the database. I have two entities as following:
public class EntityA {
String primaryKey;
#JoinColumn(
updatable = false,
insertable = false,
name = "primaryKey",
referencedColumnName = "primaryKeyOne")
List<EntityB> list;
}
public class EntityB {
String primaryKeyOne;
String primaryKeyTwo;
String name;
Integer value;
}
Now I want to create a Spring JPA Specification using these two entities.
public interface Specification<T> {
Predicate toPredicate(Root<T> root, CriteriaQuery<?> cr, CriteriaBuilder cb);
}
After I fetched EntityA, I can get the whole list of EntityB. But how do I filter out a specific instance of EntityB and create a predicate using it? My intention is as the following:
cb.gt(root.get("list").stream().filter(e -> "abc".equals(e.getName())).get("value"), 100)

Answer 1

Usually you should perform a join between the two entities and do the filtering based on the join.
Given your example, the specification will be something like the following:
public class EntityASpecifications {
public static Specification<EntityA> toSpecification(String name, Integer value){
return (root, criteriaQuery, criteriaBuilder) -> {
Join<EntityA, EntityB> entityBJoin = root.join("list");
return criteriaBuilder.and(
criteriaBuilder.equal(entityBJoin.get("name"), name),
criteriaBuilder.gt(entityBJoin.get("value"), value)
);
};
}
}
You will then use it with the repository of EntityA like so,
entityARepository.findAll(EntityASpecifications.toSpecification("abc", 100));