How to tell JSON schema validator to pick schema from property value?

Question

For example a schema for a file system, directory contains a list of files. The schema consists of the specification of file, next a sub type "image" and another one "text".

At the bottom there is the main directory schema. Directory has a property content which is an array of items that should be sub types of file.

Basically what I am looking for is a way to tell the validator to look up the value of a "$ref" from a property in the json object being validated.

Example json:

{
    "name":"A directory",
    "content":[
        {
            "fileType":"http://x.y.z/fs-schema.json#definitions/image",
            "name":"an-image.png",
            "width":1024,
            "height":800
        }
        {
            "fileType":"http://x.y.z/fs-schema.json#definitions/text",
            "name":"readme.txt",
            "lineCount":101
        }
        {
            "fileType":"http://x.y.z/extended-fs-schema-video.json",
            "name":"demo.mp4",
            "hd":true
        }

    ]
}

The "pseudo" Schema note that "image" and "text" definitions are included in the same schema but they might be defined elsewhere

{
    "id": "http://x.y.z/fs-schema.json",
    "definitions": {
        "file": {
            "type": "object",
            "properties": {
                "name": { "type": "string" },
                "fileType": {
                    "type": "string",
                    "format": "uri"
                }
            }
        },
        "image": {
            "allOf": [
            { "$ref": "#definitions/file" },
            {
                "properties": {
                    "width": { "type": "integer" },
                    "height": { "type": "integer"}
                }
            }
            ]
        },
        "text": {
            "allOf": [
            { "$ref": "#definitions/file" },
            { "properties": { "lineCount": { "type": "integer"}}}
            ]
        }
    },
    "type": "object",
    "properties": {
        "name": { "type": "string"},
        "content": {
            "type": "array",
            "items": {
                "allOf": [
                { "$ref": "#definitions/file" },
                { *"$refFromProperty"*: "fileType" } // the magic thing
                ]
            }
        }
    }
}

Answer 1

The validation parts of JSON Schema alone cannot do this - it represents a fixed structure. What you want requires resolving/referencing schemas at validation-time.

However, you can express this using JSON Hyper-Schema, and a rel="describedby" link:

{
    "title": "Directory entry",
    "type": "object",
    "properties": {
        "fileType": {"type": "string", "format": "uri"}
    },
    "links": [{
        "rel": "describedby",
        "href": "{+fileType}"
    }]
}

So here, it takes the value from "fileType" and uses it to calculate a link with relation "describedby" - which means "the schema at this location also describes the current data".

The problem is that most validators do not take any notice of any links (including "describedby" ones). You need to find a "hyper-validator" that does.

UPDATE: the tv4 library has added this as a feature