json_schema: {
      "definitions": {
          "MySubObject": {
              "_id": "filled#",
              "properties": {
                  "a": {
                      "type": "boolean"
                  }
              },
              "required": [
                  "a"
              ],
              "type": "object"
          }
      },
      "properties": {
          "empty": {
              "_id": "empty#"
          },
          "filled": {
              "$ref": "#/definitions/MySubObject"
          }
      },
      "required": [
          "empty",
          "filled"
      ],
      "type": "object"
  }