define "xref" as a "relation"

[duglin]

today it's:

"xref": {
  "name": "xref",
  "type": "url"
},

Actually, thinking more about this... if we made "type" = "relation", that makes "target" a required field. However, this is a problem because we don't know which GROUP/RESOURCE to point to. There could be any number of Groups that share the same Resource definition. Could this be an issue for "relation" in general because once we share resource definitions we really should support a "relation" pointing to any instance of that Resource type regardless of which Group it's in.

[duglin]

If we did this then:

"xref": {
  "name": "xref",
  "type": "url"
}

would change to something like (for endpoints/messages):

"xref": {
  "name": "xref",
  "type": "relation",
  "target": "/messagegroups/messages"
}

And each "xref" definition for each Resource defined in the model would be similarly customized. For example, messagegroups/messages would have - the same thing:

"xref": {
  "name": "xref",
  "type": "relation",
  "target": "/messagegroups/messages"
}

Actually this doesn't seem right and might be too restrictive since (I think) any "message" should be able to xref to any other "message" as long as all of the "message" definitions point to the same type-def via ximport. Let's discuss.

[duglin]

xref's target is always "self-xid" unless this Resource uses "ximport" then it's the target ximport's self-xid

[duglin]

@deissnerk can you look at the PR ( #239 ) and see if I got the URL/URI stuff right ?