how to get type reference from value in json schema - jsonschema

Question

I've got a json schema and I have 3 types of media, caption, image, and avatar.
Each of these media types has a different structure, so I'm using $ref and oneOf to specify which are valid options.
However, I can't figure out how to specify which ref to use based on a the value of a sibling.
My schema looks like this
const mediaSchema = {
"type": "object",
"required": ["mediaType", "content", "points"],
"properties":{
"mediaType": {"type":"string", "pattern": "^(image|avatar|caption)$"},
"content": {
"oneOf": [
{"$ref":"#/definitions/image"},
{"$ref": "#/definitions/caption"},
{"$ref": "#/definitions/avatar"}
],
}
},
"definitions": {
"caption":
{"type": "object",
"required": ["text"],
"properties": {
"text": {"type": "string"},
"fontSize": {"type": "string", "pattern": "^[0-9]{1,3}px$"}
}
},
"image": {"type": "string", "format": "url"},
"avatar":
{"type": "object",
"properties": {
"name": {"type": "string"},
"image": {"type": "string", "format":"url"}
}
}
}
}
and when I define an avatar like
mediaItem = {
"mediaType":"avatar",
"content": {
"name": "user name",
"avatar": "https://urlToImage
}
}
it should be valid, but if I define an avatar as
mediaItem = {
"mediaType": "avatar",
"content": "https://urlToImage"
}
it should throw an error as that is not valid for a media type of avatar.

Answer 1

You are on the right track, but you should put the oneOf dispatcher to the root of the schema, and define the "content" with 3 separate constants as a discriminator, like this:
{
"oneOf": [
{
"type": "object",
"properties": {
"mediaType": {
"const": "avatar"
},
"content": { "$ref": "#/definitions/avatar" }
},
"required": ["mediaType", "content"]
},
// ...
],
"definitions": {
// ...
}
}
Note: the "const" keyword exists only in the latest version of json schema (draft6). It may happen that the validator implementation you use doesn't support it yet. In that case you can replace "const": "avatar" with a single-element enum like "enum": ["avatar"]