Root level $ref and $id in Draft 7

[lud]

Hello,

In my JSON schema library a user reported a case that was not properly handled by my library. The case when $ref has a sibling $id but there is no "outer scope".

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$id": "http://example.com/schema.json",
  "$ref": "#/definitions/foo",
  "definitions": {"foo":  {"type": "object"}}
}

From what I understand from the docs, $ref should resolve against the $id because $id defines the base URI of the whole schema. Though I am not 100% sure of what should happen depending on the $ref relative URL. In my fix the above schema is now valid and the $ref resolves to http://example.com/schema.json#/definitions/foo.

I also added a test to cover what happens if there is NO base URI defined with $id. In that case, if the ref is a pointer like #/definitions/foo then everything is fine, but if the ref is something like other.json then it is a schema parse/build error, since we cannot know a base URI for other.json (whereas #/definitions/foo is fine because we can resolve against what is called :root in my lib, the root, URI-less document).

What do you think about this? What concrete tests should be added to Draft7 test suite and maybe subsequent suites?

Thank you :)

[jdesrosiers]

Whenever $ref is present, the object becomes a reference object, not a schema object. Properties that are keywords in a schema object are just plain properties in a reference object. That includes $schema and $id. Technically, you're supposed to ignore them, although I think most implementations will at least respect $schema and some will respect $id/$anchor.

What to do with definitions is a little more ambiguous. If definitions is ignored, does that mean you can't reference it? Opinions on how to interpret that vary and it was decided that the behavior would be implementation-defined. Personally, I consider a reference a scalar value and consider it an error to index into it. That means this schema wouldn't work in my implementation.

So, while using $ref at the top level might work in some validators, schema authors should never do that. The right thing to do in this situation is to wrap the reference in an allOf ("allOf": [{ "$ref": "#/definitions/foo" }]).

All of that only applies up to draft-07. Beyond draft-07, $ref works like any other keyword. Nothing is ignored and you don't need the wrapper.

[lud]

Hi :)

Personally, I consider a reference a scalar value and consider it an error to index into it.

I chose to indeed ignore $id but to honor definitions so I can just reuse the same code as 2020-12 that allows to point anywhere in the JSON document.