How do you extend json schema meta schema to support new properties? - jsonschema

Question

I want to allow a $role properties anywhere in a json schema document where type is allowed. In theory I should be able to extend it as below where I do allOf against both the json schema meta-schema and my extension for $role which includes additionalProperties to pick up my extension meta-schema recursively. What I find is that I get validation for a top-level $role, but not for any embedded one. Am I missing something? Is there some other way I should extend the JSON Schema meta-schema?
I've tried a bunch of different validators and they all fail at validating the second $role in the example.
{
"$schema": "http://json-schema.org/draft-07/schema#",
"definitions": {
"role": {
"type": "string",
"pattern": "^((lg)|(unionType\\([a-zA-Z][a-zA-Z0-9.]*\\)))$"
},
},
"allOf": [
{
"$ref": "http://json-schema.org/draft-07/schema#"
},
{
"properties": {
"additionalProperties": {
"$ref": "#"
},
"$role": {
"oneOf": [
{
"$ref": "#/definitions/role"
},
{
"type": "array",
"items": {
"$ref": "#/definitions/role"
}
}
]
}
}
}
]
}
Example using this schema:
{
"$schema": "schema.json",
"title": "Prompt",
"$role": "unionType(Microsoft.IPrompt)",
"properties": {
"prompt": {
"type": "string",
"$role":"foo"
}
}
}
What I expect is that the second role should be invalid according to the schema. What I get is that it is valid even though it does not match the $role pattern. The first $role does validate successfully.

Answer 1

Yep, extending a meta schema is more complicated than it seems. Checkout the the JSON Hyper-Schema meta schema for an example of how to extend a meta schema. JSON Hyper-Schema adds two keywords: base and `links. When extending a schema, you need to redefine any recursive reference used in the original schema.
JSON Schemas (including meta-schemas) are immutable. You can't selectively modify an existing schema. Your meta schema only validates the $role keyword, all other keywords are validated by the draft-07 meta schema. Because your meta schema doesn't modify the draft-07 schema, keywords like properties are validated entirely within the context of the draft-07 schema and without knowledge of the new keyword you added in another schema.
It's unfortunate that so much duplication is involved in extending schemas and it is a problem that is being worked on. A potential solution to make this easier is slated to be introduced in the next draft.