How to reference count index in another module? - terraform

Question

I have deployed two subnets by using count.index then I need to referece the id of subnet in subnet_route_table_association module. Can any one advise me what is the correct way to do that?
here is my code
/application/demo/main.tf
module “subnet_association” {
source = “…/…/Modules/subnet_association”
subid = var.subid
subnet_id = module.subnet.subnet_id
route_table_id = module.route_table.route_table_id
}
Modules/subnet/main.tf
resource "azurerm_subnet" "module-spoke-subnet" {
count = var.subnet_count
name = element(var.subnet_name, count.index)
resource_group_name = var.resource_group_name
virtual_network_name = var.virtual_network_name
address_prefixes = [var.subnet_address[count.index]]
enforce_private_link_endpoint_network_policies = true
enforce_private_link_service_network_policies = true
}
variable "resource_group_name" {
}
variable "virtual_network_name" {
}
Modules/subnet/output.tf
output "subnet_id" {
value = azurerm_subnet.module-spoke-subnet.*.id
}
Modules/subnet_association/main.tf
resource "azurerm_subnet_route_table_association" "module-subnet-association" {
subnet_id = var.subnet_id
route_table_id = var.route_table_id
}
variable "subnet_id" {
}
variable "route_table_id" {
}
and I`m getting the error
Error: Incorrect attribute value type
│
│ on ..\..\modules\Subnet_association\main.tf line 20, in resource "azurerm_subnet_route_table_association" "module-subnet-association":
│ 20: subnet_id = var.subnet_id
│ ├────────────────
│ │ var.subnet_id is tuple with 1 element
│
│ Inappropriate value for attribute "subnet_id": string required.

Answer 1

Output subnet_id is a list of elements. Hence, if you want to create association for each subnet try below:
resource "azurerm_subnet_route_table_association" "module-subnet-association" {
count = length(module.subnet[*].subnet_id)
subnet_id = module.subnet[count.index]. subnet_id
route_table_id = var.route_table_id
}
Or if you are creating only one subnet try below:
resource "azurerm_subnet_route_table_association" "module-subnet-association" {
subnet_id = module.subnet[0]. subnet_id
route_table_id = var.route_table_id
}