I have 3 private and 3 public subnets, and I want to create either 1 or 2 NAT Gateways depending on the environment.

So, for dev and staging there should be 1 NAT Gateway, and for production two.

Since I can’t use for_each with subnets as it is one more than I need, I decided to create the Elastic IPs with count.

resource "aws_eip" "elastic_ip" {
  count = var.environment == "stg" ? 1 : 2
  vpc = true

  tags = merge(var.tags, {
    Name        = "eip-${var.name_suffix}-${count.index}"
    Description = "Terraform Managed Elastic IP"
    Project     = var.project
    Environment = var.environment
  })
}

Now I want to create the NAT Gateways depending on the number of Elastic IPs created, like this:

resource "aws_nat_gateway" "nat_gw" {
  for_each      = aws_eip.elastic_ip
  allocation_id = each.value.id
  subnet_id     = CHALLENGE 2

  tags = merge(var.tags, {
    Name        = "nat-gw-${var.name_suffix}"
    Description = "Terraform Managed NAT Gateway"
    Project     = var.project
    Environment = var.environment
  })
}

but here I have two challenges.

1. it complaints about aws_eip.elastic_ip being a tuple. I have tried using toset(), but it didn’t work
2. I need to dynamically pull the IDs of two out of three subnets.

Is this actually possible without extra locals or variables?