Search for an element in a global context messages array in node-red

Question

Context

I defined in the context of my node-red instance a global context variable "devices" with each element defined by the {device_id, agent_id} element pair that it obtains from the values in the received message payload. Here is an example of a message and the global var:

msg.payload: {"agent_id":"agent01","device_id":"device01"}
devices: [{"agent_id":"agent01","device_id":"device01"},{"agent_id":"agent02","device_id":"device01"},{"agent_id":"agent01","device_id":"device03"}]

I would like to make it so that after receiving the message, the flow via the change and switch nodes (NO function node) would look to see if the device_id - agent_id pair is already present in the global variable, and if it is not present to add it to the array.

My attempt

I tried to define the value of a boolean format element in the "msg.exists" attribute message via a change node with the JSONata expression:

$exists($globalContext('devices')[agent_id = $.payload.agent_id and device_id = $.payload.device_id])

and then via a switch node I route the stream according to the value of msg.exist: if true I add no element, if false I add it.

The problem

However, the value of msg.exist is always equal to false, even when I supply it with values that are present in the global variable.

Could you provide a solution to my problem?

Answer 1

I belive the issue is in the filter. Try adding another $ to get the agent_id from the root object, like so:

$exists($globalContext('devices')[agent_id = $$.payload.agent_id and device_id = $$.payload.device_id])

If that doesn't work, try changing $globalContext('devices') to $.globalContext.devices.

Also, if your json input is:

{
  "payload": {
    "agent_id": "agent01",
    "device_id": "device01"
  },
  "globalContext": {
    "devices": [
      {
        "agent_id": "agent01",
        "device_id": "device01"
      },
      {
        "agent_id": "agent01",
        "device_id": "device02"
      },
      {
        "agent_id": "agent01",
        "device_id": "device03"
      }
    ]
  }
}

And your expression is:

$.globalContext.devices[agent_id = $$.payload.agent_id and device_id = $$.payload.device_id]

or this

(
  $payloadAgentId := $.payload.agent_id;
  $payloadDeviceId := $.payload.device_id;
  $.globalContext.devices[agent_id = $payloadAgentId and device_id = $payloadDeviceId]
)

The result will be truthy:

{
  "agent_id": "agent01",
  "device_id": "device01"
}