POST RAML schema, JSON format:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "definitions": {
    "uuid": {"$ref": "../_common/types/uuid.json"},
    "url": {"$ref": "../_common/types/url.json"}
  },
  "type": "object",
  "additionalProperties": false,
  "properties": {
    "domain": {
      "type": "string"
    },
    "type": {
      "type": "string",
      "enum": ["book", "car", "toy"]
    },
    "author": {
      "type": "string"
    },
    "external_id": {
      "type": "string"
    },
  },
  "required": ["domain", "type"]
}

How can I express these conditions:

• The author will be ignored for types other than books.
• The external_id will be generated for books (it should not be provided for books).

Thank you