"mediaGroup_node": {
  "type": "object",
  "additionalProperties": false,
  "required": [
    "type",
    "content"
  ],
  "properties": {
    "type": {
      "enum": ["mediaGroup"]
    },
    "content": {
      "type": "array",
      "minItems": 1,
      "items": {}
    }
  }
}

 "type": {
  "enum": ["mediaGroup"]
}

 "content": {
  "type": "array",
  "minItems": 1,
  "items": {"$ref": "#/definitions/media_node"}
}

 "items": {"$ref": "#/definitions/media_node"}