Dynamically getting values from Terraform output and pass them by index - amazon-web-services

Question

Terraform greenhorn here. I want to output and dynamically add only the first 2 subnets in the vpc_zone_identifier below. But I can't get it by index, is asking for a name.
vpc_zone_identifier = [module.subnet[0].subnet_id, module.subnet[1].subnet_id]
Here is what I tried.
Any suggestion is appreciated.
resource "aws_subnet" "this" {
vpc_id = var.vpc_id
cidr_block = var.cidr_block_subnet
map_public_ip_on_launch = var.map_public_ip_on_launch
tags = var.tags
}
module "subnet" {
source = "./aws_modules/subnet"
for_each = {for key, value in var.subnet_settings: value.cidr_block_subnet => value}
vpc_id = module.vpc.vpc_id
cidr_block_subnet = each.key
map_public_ip_on_launch = each.value.map_public_ip_on_launch
tags = var.tags
depends_on = [module.vpc]
}
variable "subnet_settings" {
type = list(object({
cidr_block_subnet = string
map_public_ip_on_launch = bool
}))
}
subnet_settings = [
{
cidr_block_subnet = "10.1.1.0/24"
map_public_ip_on_launch = false
},
{
cidr_block_subnet = "10.1.2.0/24"
map_public_ip_on_launch = false
},
{
cidr_block_subnet = "10.1.3.0/24"
map_public_ip_on_launch = false
}
]
output "subnet_id" {
value = aws_subnet.this.id
}
resource "aws_autoscaling_group" "autosys" {
desired_capacity = var.node-count
max_size = var.node-count
min_size = var.node-count
name = var.asg-name
vpc_zone_identifier = [module.subnet[0].subnet_id, module.subnet[1].subnet_id]
launch_template {
id = aws_launch_template.autosys.id
version = "$Latest"
}
}

Answer 1

In a for expression, you can initialize two variables within the scope of the lambda when iterating on a list type. In this situation, the first variable will be assigned the index of the iteration. Therefore, we can modify the for_each meta-argument of your module:
module "subnet" {
source = "./aws_modules/subnet"
for_each = {for idx, subnet in var.subnet_settings: idx => subnet}
vpc_id = module.vpc.vpc_id
cidr_block_subnet = each.value.cidr_block_subnet
map_public_ip_on_launch = each.value.map_public_ip_on_launch
tags = var.tags
depends_on = [module.vpc]
}
and now the module key will be the index of the var.subnet_settings. Correspondingly, the module output namespace will now be module.<declared_name>["<index>"].<output_name>. Therefore, we can modify the value for the vpc_zone_identifier argument to have keys of string type instead of number type:
vpc_zone_identifier = [module.subnet["0"].subnet_id, module.subnet["1"].subnet_id]
and now the passed value to the argument will be the first two subnets as desired.