Syntax for filtering by key existence (equivalent to @..key)

[djs0109]

Hi!

I am trying to filter a JSON array to select objects that match a specific attribute and possess a specific key.

In standard JSONPath (RFC 9535), I can achieve this using the existence check syntax ?(@..key). Here is an example dataset

[
  {
    "id": "room1",
    "type": "HotelRoom",
    "hasLocation": {
      "type": "Relationship",
      "value": "fiware_hotel",
      "metadata": {}
    },
    "name": {
      "type": "Text",
      "value": "fiware_room_base_1_v2",
      "metadata": {}
    },
    "temperature": {
      "type": "Number",
      "value": 0.0,
      "metadata": {}
    }
  },
  {
    "id": "room2",
    "type": "HotelRoom",
    "hasLocation": {
      "type": "Relationship",
      "value": "fiware_hotel",
      "metadata": {}
    },
    "name": {
      "type": "Text",
      "value": "fiware_room_base_1_v2",
      "metadata": {}
    },
    "co2": {
      "type": "Number",
      "value": 0,
      "metadata": {}
    }
  }
]

In this case, I want to select the room2, which has the key co2. I can use $[?(@.type == "HotelRoom" && @.co2)] to achieve it. You can test it here.

However, since this library uses a specific expressions for filtering, I am struggling to find the equivalent syntax to check if a key exists within the current object. I have tried $[?(@.type == "HotelRoom" and @.co2)], but it does not work. Could you give me any suggestion or maybe this is a not supported feature?

Thanks!

[sean2077]

This already works with the and operator for truthiness-based key existence checks:

data = [
    {"id": "room1", "type": "HotelRoom", "temperature": {"type": "Number"}},
    {"id": "room2", "type": "HotelRoom", "co2": {"type": "Number", "value": 0}},
]
JSONPath('$[?(@.type == "HotelRoom" and @.co2)]').parse(data)
# [{'id': 'room2', 'type': 'HotelRoom', 'co2': {'type': 'Number', 'value': 0}}]

The syntax @.co2 in a filter evaluates the value at key co2. If the key doesn't exist, the expression raises a KeyError which is treated as False. If it exists and has a truthy value (non-empty dict, non-zero number, etc.), it's True.

Note: This uses Python truthiness, not strict existence — keys with falsy values (0, None, "", [], False) would not match. For the example data in this issue, co2 has a dict value which is truthy, so it works correctly.

Use and instead of && (Python syntax, not JavaScript).