Terraform - Add droplet to Firewall

Hi, I’m migrating the management of the infrastructure to Terraform.

I need to write a script that

• Create a droplet
• Add the droplet to an existing firewall.

From the documentation, digitalocean_firewall accepts droplet_ids, a list of ids of droplets. My problem is that I should add the new droplet id without removing the others.

I’ve tried creating a

data "digitalocean_firewall" "default" {
  firewall_id = var.firewall_id
}

and then using concat

resource "digitalocean_firewall" "droplet-firewall" {
  name = "Default"
  droplet_ids = concat(data.digitalocean_firewall.default.droplet_ids, [digitalocean_droplet.droplet.id])
}

Before running this I’ll run terraform import to add the resource

But I get an error about data.digitalocean_firewall.default.droplet_ids is null and I don’t know if it’s the correct method to do this.

Thank you