Add existence flag to relationships [DATAREST-1422]

Dario Tortola opened DATAREST-1422 and commented

As-is: lets say we have an entity A related x to one with an entity B, so that A has a B b field

A { private B b; }

When A is converted to a resource by Spring Data Rest, that result will include the link to b

{ links : { b : { href: "host:port/path/b" }}}

Now, the relationship b is optional, so accessing that url returns 404, so the developer needs to account for that possible error and they won't know if there is a resource until they try, incurring both in communication time and exception-based programming.

Desired: if Link could have a "boolean present" field, the developer would know without an additional request. It could be a "Boolean present" to allow for "yes; no; I-don't-know-you-try". If Link is not modifiable could be wrapped or if final this information could be in a similar object

{ links : { b : { href: "host:port/path/b" }}, linkPresence : { b : false }}

Considering that this is only for x-to-one relationships, they would incur in additional database cost only if the relationship is lazy-loaded (which could be transmitted with the Boolean present = null).

If lazy-loaded, there's possibly a way to check if it has been initialized or otherwise to fill present with true|false.

Of course, the same consideration should be used with projections, considering that @JsonIgnored relationships wouldn't need this flag, since they aren't sent, etc

No further details from DATAREST-1422