Traverson cannot follow multiple links with same rel

[asgeirn]

Similar to traverson/traverson#7, TraversalBuilder cannot follow multiple links from a resource having the same rel value:

{
    "_links": {
        "parorende": [{
                "href": "https://play-with-fint.felleskomponent.no/utdanning/elev/kontaktperson/systemid/XXXXXX"
            },
            {
                "href": "https://play-with-fint.felleskomponent.no/utdanning/elev/kontaktperson/systemid/YYYYYY"
            }
        ],
        "self": [{
            "href": "https://play-with-fint.felleskomponent.no/utdanning/elev/person/fodselsnummer/02040754515"
        }]
    }
}

[gregturn]

Intriguing problem.

What would you propose as the how the API should look to handle this?

[asgeirn]

There are discussions over at traverson/traverson-hal#7, but the feature hasn't fully landed in traverson just yet.

I would suggest an API that leverages java.util.Stream, perhaps using method signatures like

Stream<T> toObjects(Class<T> cls)

The tricky part is if an intermediate link in the traversal chain is plural, as this means the traversal process needs to be recursive in able to resume traversal at an earlier stage after completing one path.

This could for instance be indicated using .follow("rel[]") or .follow("rel[$all]") as in traverson.

[gregturn]

Frankly I don’t grok a relationship with multiple links. How do you present that to the user?

Multiple rels, I get. But not the other way around.

[schauder]

I guess one could have a button/link that tries the first and if that fails tries the next.

[asgeirn]

What we're trying to express is in fact multiple links, to different resources, of the same relation. The data in the example above attempts to express next of kin, in this concrete example parents, but it does not have to be parents in the traditional interpretation. In a modern world, parents are no longer exactly two individuals of opposite gender, hence the requirement for a more general way of expressing relationships.

Quoting from http://stateless.co/hal_specification.html:

Representing Multiple Links With The Same Relation

A resource may have multiple links that share the same link relation.

For link relations that may have multiple links, we use an array of links.

 {
      "_links": {
       "items": [{
           "href": "/first_item"
       },{
           "href": "/second_item"
       }]
     }
 }

Note: If you're unsure whether the link should be singular, assume it will be multiple. If you pick singular and find you need to change it, you will need to create a new link relation or face breaking existing clients.

[gregturn]

Okay I’ve seen recent hypermedia showing multiple links tied to one link relation.

Sounds like Traverson would need a strategy like FIRST, RANDOM, FAILOVER, ???

What do you think?