First at all I read the previous question: Exposing link on collection entity in spring data REST
But the issue still persist without trick.
Indeed if I want to expose a link for a collections resources I'm using the following code:
@Component
public class FooProcessor implements ResourceProcessor<PagedResources<Resource<Foo>>> {
private final FooLinks fooLinks;
@Inject
public FooProcessor(FooLinks fooLinks) {
this.FooLinks = fooLinks;
}
@Override
public PagedResources<Resource<Foo>> process(PagedResources<Resource<Foo>> resource) {
resource.add(fooLinks.getMyCustomLink());
return resource;
}
}
That works correctly except when collection is empty...
The only way to works is to replace my following code by:
@Component
public class FooProcessor implements ResourceProcessor<PagedResources> {
private final FooLinks fooLinks;
@Inject
public FooProcessor(FooLinks fooLinks) {
this.FooLinks = fooLinks;
}
@Override
public PagedResources process(PagedResources resource) {
resource.add(fooLinks.getMyCustomLink());
return resource;
}
}
But by doing that the link will be exposed for all collections.
I can create condition for exposing only for what I want but I don't think is clean.
I think spring does some magic there trying to discover the type of the collection - on an empty collection you cannot tell which type it is of - so spring-data-rest cannot determine which ResourceProcessor to use.
I think I have seen in
org.springframework.data.rest.webmvc.ResourceProcessorHandlerMethodReturnValueHandler.ResourcesProcessorWrapper#isValueTypeMatch
that they try to determine the type by looking at the first element in the collection and otherwise just stop processing:
if (content.isEmpty()) {
return false;
}
So I think you cannot solve this using spring-data-rest. For your controller you could fall back to writing a custom controller and use spring hateoas and implement your own ResourceAssemblerSupport to see the link also on empty collections.
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