DTO在递归结构上的投影

Question

In my data model there is an entity "location" which is recursively. Furthermore there are relations to other entities.

The corresponding JPA (Spring Data JPA) entity looks like:

@Entity
@Table(name = "location")
class Location{

  @OneToMany(mappedBy = "parent", orphanRemoval = true)
  @OrderBy("name ASC")
  Set<Location> children = null

  @ManyToOne(fetch = FetchType.EAGER)
  @JoinColumn(name = "parent_id")
  Location parent = null

  @Column(name = "name")
  String name = null

  @OneToMany(mappedBy = "location", fetch = FetchType.EAGER)
  Stops stops = null
  ...

What is the most performant way to do a read only query? I just need the information inside the entity (table location) with the complete recursive structure but no information from the related entities.

I've read the phrase DTO projection, but nothing about what to do with a recursive structure.

Answer 1

Reading a recursive structure is usually done by making use of what SQL calls a recuresive CTE . JPA does not support that out of the box, because not all RDBMS support it. If you know that your DBMS supports it, you can make use of the following SQL to do this:

WITH RECURSIVE nodes(id, parent_id) AS (
  SELECT id, parent_id FROM location l where id = ?
  UNION ALL
  SELECT l.id, l.parent_id FROM nodes n JOIN location l ON n.parent_id = l.id
)
SELECT id, parent_id FROM nodes

With that you get a list of a specific and all parent location ids as well as their respective parents which is flat. You will have to bring structure into this.

List<Object[]> result = //get the result of the query
Map<Integer, LocationDto> locationMap = new HashMap<>();
result.forEach(r -> locationMap.put(result.get(0), new LocationDto(result[0], result[1])));
locationMap.values().forEach(l -> l.setParent(locaitonMap.get(l.getParentId())));

If you don't want to make use of plain SQL because of portability concerns or just because you don't want to give up on your abstraction, you can make use of Blaze-Persistence which works on top of JPA and adds support for CTEs. Your query with blaze-persistence would look like this

List<LocationCte> result = criteriaBuilderFactory.create(entityManager, LocationCte.class)
  .withRecursive(LocationCte.class)
    .from(Location.class, "l")
    .bind("id").select("l.id")
    .bind("parent").select("l.parent.id")
    .where("id").eq(initialId)
  .unionAll()
    .from(Location.class, "l")
    .innerJoinOn(LocationCte.class, "cte")
      .on("cte.parent").eqExpression("l.id)
    .end()
    .bind("id").select("l.id")
    .bind("parent").select("l.parent.id")
  .end()
  .from(LocationCte.class)
  .getResultList();

You will also need this special entity class

@CTE
@Entity
public class LocationCte {
  @Id Integer id;
  Integer parent;
}