"All other properties in a "$ref" object MUST be ignored." - so can't "subclass" a schema?

[Vladimir Dyuzhev]

Hello, folks. Newbie here.

Draft 7 "8.3 Schema references with $ref" says that if a schema object has $ref, all other properties MUST be ignored.

That made me pause. I imagine it can be useful to create a base schema #abstract-base and then override some of its properties, e.g. add numerical restrictions:

"wholesale": {

  "$ref": "#/definitions/abstract-base",

  "minimum": 20

},

"retail": {

  "$ref": "#/definitions/abstract-base",

  "minimum": 1,

  "exclusiveMaximum": 20,

}

Where can I learn the reasoning for the strict $ref?

Also, since above subclassing is not valid, can the same effect be achieved in some other way (save for copy-pasting abstract-base properties into both "subclassed" objects)?

Thanks!

[Ben Hutton]

Hi Vladimir,

 

No one currently working on the project is really sure WHY.

We have changed this for draft-8!

 

See https://github.com/json-schema-org/json-schema-spec/pull/628

 

 

Cheers

 

Ben

(JSON Schema team)

-- The Wellcome Sanger Institute is operated by Genome Research Limited, a charity registered in England with number 1021457 and a company registered in England with number 2742969, whose registered office is 215 Euston Road, London, NW1 2BE.

[Austin William Wright]

The reason is for historical reasons: objects with a $ref property aren't considered schemas, but instead they're a "JSON Reference", which used to be a separate, re-usable specification. This wasn't a good idea for several reasons, so in a removed the reference, but had to keep the same behavior: Since other properties in these objects weren't considered schemas, they're ignored.

As of recently, JSON Schema no longer has this restriction, however for compatibility, I would put $ref references inside an allOf statement:

"wholesale": {

 "allOf": [ { "$ref": "#/definitions/abstract-base" } ],

 "minimum": 20

},

One more note, when you add keywords like this, what you're doing is creating a subset, which sometimes isn't quite the same thing as a subclass, depending on how you're going about it.

[Henry Andrews]

> One more note, when you add keywords like this, what you're doing is creating a subset, which sometimes isn't quite the same thing as a subclass, depending on how you're going about it.

Yes, this is a key point.  JSON Schema is a constraint system, not a definition system.  In a definition system, the empty object does nothing.  In a constraint system, the empty schema allows everything.

thanks,

-henry

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