Spring Data Rest adding excerpt projection switches off lazy fetching - loading

Question

I'm new to Spring Data Rest and trying to play around with its basic concepts. Everything works well so far, but few days ago I noticed that the application performance suddenly dropped after putting the projections into business.
These are my entities, repositories and the projection
#Entity
public class Item {
#Id
#GeneratedValue(strategy = TABLE)
private long id;
private String code;
private String name;
#ManyToOne(targetEntity=Category.class)
#JoinColumn(name="category_id", referencedColumnName="id")
private Category category;
//getters & setters
}
#Entity
public class Category {
#Id
#GeneratedValue(strategy = TABLE)
private long id;
private String name;
#OneToMany(mappedBy="category", targetEntity=Item.class, fetch=FetchType.LAZY)
private Set<Item> items;
//getters & setters
}
#RepositoryRestResource(excerptProjection=ItemExcerpt.class)
public interface ItemRepository extends CrudRepository<Item, Long>{
}
#RepositoryRestResource
public interface CategoryRepository extends CrudRepository<Category, Long>{
}
#Projection(name="excerpt", types=Item.class)
public interface ItemExcerpt {
String getName();
}
So, all worked fine untill I added the excerpt projection to the ItemRepository #RepositoryRestResource(excerptProjection=ItemExcerpt.class)
Before doing this, when I hit http://localhost:9191/categories Hibernate output was as I expected it to be:
select
category0_.id as id1_0_,
category0_.name as name2_0_
from
category category0_
This is the output that I get after adding excerptProjection=ItemExcerpt.class
Hibernate:
select
category0_.id as id1_0_,
category0_.name as name2_0_
from
category category0_
Hibernate:
select
items0_.category_id as category4_1_0_,
items0_.id as id1_1_0_,
items0_.id as id1_1_1_,
items0_.category_id as category4_1_1_,
items0_.code as code2_1_1_,
items0_.name as name3_1_1_
from
item items0_
where
items0_.category_id=?
My conclusion is that the excerpt projection makes lazy fetching being ignored on #OneToMany relationship, which leads to a performance drop.
Does anyone know a way to bypass this issue, or is this maybe an expected behaviour?

Answer 1

It isn't exactly that excerpt projections make lazy fetching be ignored. More specifically, it is that an excerpt projection is telling spring data to include the excerpted data wherever a collection resource would be returned.
From the reference docs Projections Excerpts, "An excerpt is a projection that is applied to a resource collection automatically.". The unfortunate side-effect of this is that spring-hateoas then ignores that property and instead puts in the hypermedia link to the resource instead. There is no combination of annotations that i have found that will correct this behavior for you while preserving the output. #JsonIgnore will not prevent the extra queries. #RestResource(exported = false) will prevent the queries, but will also prevent the hypermedia link.