how to reference objects in terraform - azure

Question

I am creating an Azure VNet using terraform, and creating a couple subnets in it. Later on , I want to create a network interface, and want to put it in one of the subnets already created for VNet. I do not know how to reference that subnet.
I tried below but it is now working:
subnet_id = "${azurerm_virtual_network.virtual-network.subnet.ServersSubnet.id}"
resource "azurerm_virtual_network" "virtual-network" {
name = "${var.ClientShortName}-az-network"
address_space = ["${local.AzureInfraNetwork}"]
location = "${var.resource-location}"
resource_group_name = "${azurerm_resource_group.test-resource-group.name}"
subnet {
name = "ServersSubnet"
address_prefix = "${local.ServersSubnet}"
}
subnet {
name = "GatewaySubnet"
address_prefix = "${local.GatewaySubnet}"
}
}
Error: Cannot index a set value
on main.tf line 120, in resource "azurerm_network_interface" "DCNIC":
120: subnet_id = "${azurerm_virtual_network.virtual-network.subnet.ServersSubnet.id}"
Block type "subnet" is represented by a set of objects, and set elements do
not have addressable keys. To find elements matching specific criteria, use a
"for" expression with an "if" clause.

Answer 1

Below is the complete solution.
If the subnets are created as blocks, you can reference a given subnet's resource ID as follows:
resource "azurerm_resource_group" "main" {
name = "vnet-rg"
location = "eastus"
}
resource "azurerm_virtual_network" "main" {
name = "my-vnet"
location = azurerm_resource_group.main.location
resource_group_name = azurerm_resource_group.main.name
address_space = ["10.0.0.0/16"]
subnet {
name = "subnet1"
address_prefix = "10.0.1.0/24"
}
subnet {
name = "subnet2"
address_prefix = "10.0.2.0/24"
}
subnet {
name = "subnet3"
address_prefix = "10.0.3.0/24"
}
}
output "subnet1_id" {
value = azurerm_virtual_network.main.subnet.*.id[0]
}
output "subnet2_id" {
value = azurerm_virtual_network.main.subnet.*.id[1]
}
output "subnet3_id" {
value = azurerm_virtual_network.main.subnet.*.id[2]
}

Answer 2

When creating subnets as blocks you must reference them using the list syntax, e.g.:
foo = azurerm_virtual_network.virtual-network.subnet[0].id
bar = azurerm_virtual_network.virtual-network.subnet[1].id
This is useful if the subnets form a pool of redundant resources and you don't care about referencing any subnet in particular.
I don't believe that's your case, so you might consider creating your subnets as separated resources, e.g:
resource "azurerm_virtual_network" "main" {
name = "${var.ClientShortName}-az-network"
address_space = [local.AzureInfraNetwork]
location = var.resource-location
resource_group_name = azurerm_resource_group.test-resource-group.name
}
resource "azurerm_subnet" "server" {
virtual_network_name = azurerm_virtual_network.main.name
name = "ServersSubnet"
address_prefix = local.ServersSubnet
}
resource "azurerm_subnet" "gateway" {
virtual_network_name = azurerm_virtual_network.main.name
name = "GatewaySubnet"
address_prefix = local.ServersSubnet
}
Then you could reference one of your subnets using the regular object attribute syntax:
foo = azurerm_subnet.server.id
Also note that I'm using terraform => 0.12 syntax, so I can write foo.bar instead of "${foo.bar}" when I don't need string interpolation.

Answer 3

If anyone else needs the answer to this use this:
"${element(azuread_application.events_backend.app_role[*].id,0)}"

Answer 4

You can break this out into creating the virtual network and the subnet as separate resources. Advantage of this is that you can return your subnets as a map rather than a list, making it easier to retrieve by name later on, it also makes it stable if you need to add/remove subnets at a later stage.
locals {
subnets = {
Servers = "10.0.1.0/24",
Gateway = "10.0.2.0/24"
}
}
resource "azurerm_resource_group" "main" {
name = "vnet-rg"
location = "eastus"
}
resource "azurerm_virtual_network" "main" {
name = "my-vnet"
location = azurerm_resource_group.main.location
resource_group_name = azurerm_resource_group.main.name
address_space = [ local.AzureInfraNetwork ]
}
resource "azurerm_subnet" "main" {
for_each = var.subnets
name = "${each.key}-subnet"
resource_group_name = azurerm_resource_group.main.name
virtual_network_name = azurerm_virtual_network.main.name
address_prefixes = [ each.value ]
}
output "subnets" {
value = azurerm_subnet.main
}