Query result mapping

Query results can be mapped to objects in a couple of ways.

Note that currently only SPARQL queries are supported.

TypedQuery Result Mapping

Entity Mapping

By default, TypedQuery results are directly mapped to entities. The query should select only the entity IRI, JOPA will take care of loading the corresponding entity based on the query result type.

A find all query can thus look like this:

List<Person> = em.createNativeQuery("SELECT ?x WHERE { ?x a foaf:Person . }", Person.class).getResultList();

Other Types

TypedQuery results can be also mapped to other, non-entity types. This mainly applies to primitive type wrappers, String or identifier types - URI and URL.

An example is a ASK query result mapping:

Boolean exists = em.createNativeQuery("ASK { ?x a foaf:Person . }", Boolean.class).getSingleResult();

SPARQL Result Set Mapping

JOPA also allows to map query results to non-entity objects or provide a custom mapping where, for example, object attributes can be fetched directly in the query. This is achieved using the SparqlResultSetMapping annotation, which works similarly to SqlResultSetMapping in JPA.

For instance, using a constructor mapping, an object can be constructed from the query projection variables by using the following mapping:

@SparqlResultSetMapping(name = "RdfsResource", 
   classes = {@ConstructorResult(targetClass = RdfsResource.class,
                                 variables = {                                                                   
                  @VariableResult(name = "uri", type = URI.class),
                  @VariableResult(name = "label", type = String.class),
                  @VariableResult(name = "comment", type = String.class)})})

A corresponding constructor must exist in the target class:

public RdfsResource(URI uri, String label, String comment) {
        this.uri = uri;
        this.label = label;
        this.comment = comment;
    }

Other means of using SparqlResultSetMapping include mapping to entities and variable mapping, again, similar to JPA. More info can be found in Javadoc.