HQL recursion, how do I do this?

Question

I have a tree structure where each Node has a parent and a Set children. Each Node has a String title, and I want to make

Answer 1

You can't do recursive queries with HQL. See this. And as stated there it is not even standard SQL. You have two options:

• write a vendor-specific recursive native SQL query

• make multiple queries. For example:

  // obtain the first node using your query
  while (currentNode.parent != null) {
     Query q = //create the query
     q.setParameter("id", currentNode.getParentId());
     Node currentNode = (Node) q.getSingleResult();
     nodes.add(currentNode); // this is the Set
  }
  

I'd definitely go for the 2nd option.

Answer 2

I know this question is old, but as it was linked in a different question, I wanted to give an update on this, as Blaze-Persistence offers support for working with recursive CTEs on top of the JPA model.

Blaze-Persistence is a query builder on top of JPA which supports many of the advanced DBMS features on top of the JPA model. To model CTEs or recursive CTEs, which is what you need here, you first need to introduce a CTE entity that models the result type of the CTE.

@CTE
@Entity
public class NodeCTE {
  @Id Integer id;
}

A query for your example could look like the following

List<String> titles = criteriaBuilderFactory.create(entityManager, String.class)
  .withRecursive(NodeCTE.class)
    .from(Node.class, "n1")
    .bind("id").select("n1.id")
    .where("n1.id").eq(nodeId)
  .unionAll()
    .from(Node.class, "n2")
    .innerJoinOn(NodeCTE.class, "cte")
      .on("cte.id").eq("n2.parent.id")
    .end()
    .bind("id").select("n2.id")
  .end()
  .from(Node.class, "n")
  .select("n.title")
  .where("n.id").in()
    .from(NodeCTE.class, "c")
    .select("c.id")
  .end()
  .getResultList();

This renders to SQL looking like the following

WITH RECURSIVE NodeCTE(id) AS (
    SELECT n1.id
    FROM Node n1
    WHERE n1.parent_id = :id
  UNION ALL
    SELECT n2.id
    FROM Node n2
    INNER JOIN NodeCTE cte ON n2.parent_id = cte.id
)
SELECT n.title
FROM Node n
WHERE n.id IN (
  SELECT c.id
  FROM NodeCTE c
)

You can find out more about recursive CTEs in the documentation: https://persistence.blazebit.com/documentation/core/manual/en_US/index.html#recursive-ctes

Answer 3

While it isn't possible to write the recursive query you're asking for, it is possible to eager fetch the hierarchy with HQL; doing this would at least allow you to walk the tree in memory without hitting the database for each level.

select n from Node n
left join fetch n.Children