12 September 2010

JPA 2.0: Ordered Collections

In earlier posts, I wrote about problems with persistent maps, a new feature introduced in JPA 2.0. Ordered collections with explicit order columns are another JPA 2.0 feature which, again, is not yet fully robust in all JPA 2.0 compliant and TCK tested implementations.

I'm currently working with a simple JPA model for OpenStreetMap (OSM). There are ways and nodes, and each way has a sequence of nodes. Of course the order of the nodes is important, and a node may be used more than once for a given way.

Thus, the nodes collection has to be a List, not a Set, and we need an explicit order column specifying the sequence number of the nodes along the way. Sorting the nodes by ID would not make any sense, obviously.

Here is a snippet from the model:

@Table(name = "ways")
public class OsmWay
    private long id;
    @ManyToMany(cascade = CascadeType.ALL)
    @OrderColumn(name = "sequence_id")
    @JoinTable(name = "way_nodes", 
               joinColumns = @JoinColumn(name = "id"), 
               inverseJoinColumns = @JoinColumn(name = "node_id"))
    private List<OsmNode> nodes = new ArrayList<OsmNode>();

This entity is represented by two tables:

  id bigint NOT NULL

CREATE TABLE way_nodes
  id bigint NOT NULL,
  node_id bigint NOT NULL,
  sequence_id integer,

I've tested this scenario on Hibernate 3.5.3, Eclipselink 2.1.1 and OpenJPA 2.0.1: Hibernate and OpenJPA pass, Eclipselink fails with 2 issues.

First, the generated DDL is incorrect: There is a PRIMARY KEY (id, node_id) for way_nodes. This should be (id, sequence_id).

Second, the order of the list items is not maintained in all contexts. It is ok when the collection is lazily loaded, e.g.

OsmWay way = em.find(OsmWay.class, wayId);
List<OsmNode> nodes = way.getNodes();

but it is broken when using a fetch join:

String jpql = "select distinct w from OsmWay w join fetch w.nodes";
TypedQuery<OsmWay> query = em.createQuery(jpql, OsmWay.class);
List<OsmWay> ways = query.getResultList();
OsmWay way = ways.get(0);
List<OsmNode> nodes = way.getNodes();

The funny thing is, the nodes are neither ordered by sequence_id nor by node_id. In my test case, the way has 5 nodes, one of which appears twice in different positions.

So here is some more evidence for my claim that the JPA 2.0 TCK is insufficient.

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