Reference to an array

[petermarcoen]

How do I allow a reference to an array in my schema?
For example, say I have an object that contains multiples of an array of numbers:

{
  "multiplesOf": [
    {
      "of": 1,
      "values": [1,2,3,4,5],
      "results": [1,2,3,4,5]
    },
    {
      "of": 2,
      "values": [1,2,3,4,5],
      "results": [2,4,6,8,10]
    }
  ]
}

I would write the following schema for this:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "properties": {
    "multiplesOf": {
      "type": "array",
      "items": {
        "type": "object",
        "properties": {
          "of": {
            "type": "number"
          },
          "values": {
            "type": "array"
          },
          "results": {
            "type": "array"
          }
        }
      }
    }
  }
}

That validates fine, but suppose in my data I want to avoid writing the same values twice, so I seperate them into a seperate definition:

{
  "multiplesOf": [
    {
      "of": 1,
      "values": { "$ref": "#/values" },
      "results": [1,2,3,4,5]
    },
    {
      "of": 2,
      "values": { "$ref": "#/values" },
      "results": [2,4,6,8,10]
    }
  ],
  "values": [1,2,3,4,5]
}

Then every validator I tried gives me the following error: "Invalid type. Expected Array but got Object.".
In a way this is correct since the reference itself is an object.
Is there any way to allow this?

[notEthan]

$ref is only defined by json schema for schemas - not for schema instances.

json schema doesn't specify any kind of reference syntax within json schema instances, but at least one other spec does. JSON Reference draft-pbryan-zyp-json-ref-03 covers any json document. big caveat, that draft is long out of date, but a quick googling finds at least one implementation, json-spec in python still appears to support it.

[petermarcoen]

Hi @notEthan, thanks for replying!
I also just found out that JSON parsers do not support the $ref in a JSON data document.
So, if I understand correctly, right now there is no standard way to reference something in a JSON data document, no way to avoid duplication?
I'm kind of surprised that we can't use $ref there.

[handrews]

@petermarcoen JSON Schema only specifies how JSON Schema works. Regular JSON data is define by the JSON RFC. In the past, there was an effort at $ref as its own spec (JSON Reference) but that had its own problems so it has been abandoned. There are still tools (e.g. json-schema-ref-parser in JavaScript, which despite the name works beyond JSON Schema) which will work with $ref in any JSON document, but it's not necessarily standardized.

Regardless, this is outside of the scope of JSON Schema so I'm closing it here.

[jdesrosiers]

@petermarcoen Hyperjump Validation does what you are looking for. I defined (not formally) a media type I call JRef using the old JSON Reference spec as a starting point. The Hyperjump Browser is an implementation in JavaScript. I've also created Hyperjump Validation which is very similar to JSON Schema but with several different design choices. One of the differences being that it can validate JRef instead of just JSON.

Fair warning, this was all built for experimentation, not necessarily for production use. Hyperjump Browser (JRef client) is pretty stable. Hyperjump Validation is still quite incomplete, but it wouldn't take much to make a JSON Schema compatible vocabulary. I intend to do that at some point, but it probably won't happen soon.

This isn't the right place for this discussion, but if you want to know more, I'd happy to chat in the JSON Schema slack or somewhere else.

JRef Spec: https://github.com/hyperjump-io/browser/blob/master/lib/json-reference/README.md
Hyperjump Browser: https://github.com/hyperjump-io/browser
Hyperjump Validation: https://github.com/hyperjump-io/validation
Online Validator: https://validate.hyperjump.io/

[MikeRalphson]

Similarly off-topic, but if you just want to use reference-like semantics in your JSON documents, you might look at YAML as an intermediate or alternative format. It supports anchors and aliases, and when parsed, aliased objects will still be validatable by your JSON Schema documents.

[handrews]

@MikeRalphson @jdesrosiers thanks yeah these are good alternatives to consider in addition to something like json-schema-ref-parser