Collections in QueryDSL projections - collections

Question

I am trying to use a projection to pull in data from an entity an some relations it has. However. The constructor on the projection takes three arguments; a set, integer and another integer. This all works fine if I don't have the set in there as an argument, but as soon as I add the set, I start getting SQL syntax query errors.
Here is an example of what I'm working with...
#Entity
public class Resource {
private Long id;
private String name;
private String path;
#ManyToOne
#JoinColumn(name = "FK_RENDITION_ID")
private Rendition rendition;
}
#Entity
public class Document {
private Long id;
private Integer pageCount;
private String code;
}
#Entity
public class Rendition {
Long id;
#ManyToOne
#JoinColumn(name="FK_DOCUMENT_ID")
Document doc;
#OneToMany(mappedBy="rendition")
Set<Resource> resources;
}
public class Projection {
#QueryProjection
public Projection(Set<Resource> resources, Integer pageCount, String code) {
}
}
Here is the query like what I am using (not exactly the same as this is a simplified version of what I'm dealing with)....
QRendition rendition = QRendition.rendition;
Projection projection = from(rendition)
.where(rendition.document().id.eq(documentId)
.and(rendition.resources.isNotEmpty())
.limit(1)
.singleResult(
new QProjection(rendition.resources,
rendition.document().pageCount,
rendition.document().code));
This query works fine as long as my projection class does not have the rendition.resources in it. If I try and add that in, I start getting malformed SQL errors (it changes the output sql so that it starts with this.
select . as col_0_0_
So, I guess my main question here is how do I include a Set as an object in a projection? Is it possible, or am I just doing something wrong here?

Answer 1

Using collections in projections is unreliable in JPA. It is safer to join the collection and aggregate the results instead.
Querydsl can also be used for result aggregation http://www.querydsl.com/static/querydsl/3.2.0/reference/html/ch03s02.html#d0e1799
In your case something like this
QRendition rendition = QRendition.rendition;
Projection projection = from(rendition)
.innerJoin(rendition.document, document)
.innerJoin(rendition.resources, resource)
.where(document.id.eq(documentId))
.limit(1)
.transform(
groupBy(document.id).as(
new QProjection(set(resources),
document.pageCount,
document.code)));