JPA how to aggregate columns in a join - jpa

Question

I'm trying to implement a query like this in JPA:
SELECT
ta.field_aggr_1,
ta.field_aggr_2,
MIN(tb.date_inv) AS min_date_inv,
MAX(tb.date_inv) AS max_date_inv
FROM table_a ta
INNER JOIN table_b tb ON ta.idB = tb.id
GROUP BY
ta.field_aggr_1,
ta.field_aggr_2
The key point is the MIN and MAX functions that apply in one column of a joined table.
I've created the entities:
#Entity
#Table(name="table_a")
public class EntityA extends Serializable {
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
#Column(name="field_aggr_1")
private String field_aggr_1;
#Column(name="field_aggr_2")
private String field_aggr_2;
#ManyToOne
#JoinColumn(name="idB")
private EntityB entityB;
// Getters & Setters & HashCode & equals & toString
}
#Entity
#Table(name="table_b")
public class EntityB extends Serializable {
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
#Column(name="date_inv")
private String date_inv;
// Getters & Setters & HashCode & equals & toString
}
And in the service I want to create query:
EntityManager em = ...;
Root<EntityA> root = criteriaQuery.from(EntityA.class);
EntityType<EntityA> type = this.em.getMetamodel().entity(EntityA.class);
Join<EntityA, EntityB> join = root.join(type.getDeclaredSingularAttribute("entityB", EntityB.class));
List<Selection<?>> fields = new ArrayList<Selection<?>>();
// grouping fields
fields.add(root.<EntityA>get("field_aggr_1"));
fields.add(root.<EntityA>get("field_aggr_2"));
I've managed to include fields from the joined table,
fields.add(join.<EntityB>get("date_inv"));
BUT I haven't succeed in implement the min aggregation.
Thanks in advance for your answers!

Answer 1

I have managed to solve the question. First, I needed to have one more "root" and one more "entityType" for the joined entity:
Root<EntityB> rootB = criteriaQuery.from(EntityB.class);
EntityType<EntityB> typeB = this.em.getMetamodel().entity(EntityB.class);
With these, now I can do what I needed:
fields.add(builder.least(rootB.get(typeB.getDeclaredSingularAttribute("date_inv", String.class))));
fields.add(builder.greatest(rootB.get(typeB.getDeclaredSingularAttribute("date_inv", String.class))));
Hope that it helps someone!