Get resources based on a value created using count - terraform

Question

I am using Terraform v12.19 with the aws provider v2.34.0.
Imagine, I have a resource generated with a count value:
resource "aws_iam_role" "role" {
count = length(var.somevariable)
name = var.somevariable[count.index]
}
Later on, I want to reference one specific resource instance in that way, e. g.:
resource "aws_iam_role_policy_attachment" "polatt" {
role = aws_iam_role.role["TheRoleNameIWant"].id
policy_arn = "arn:aws:iam::aws:policy/..."
}
I don't know the index, I can just rely on the name, provided by the variable. Thats because the values of the variable are provided by an external source and the order could change...
Any ideas how to do this?

Answer 1

You should be able to accomplish this using the index terraform function.
Here's a minimal example using null_resources to test it out
locals {
role_names = [
"role-a",
"role-b",
"role-c",
"role-d",
]
target_role_name = "role-c"
}
resource "null_resource" "hi" {
count = length(local.role_names)
}
output "target_resource" {
value = null_resource.hi[index(local.role_names, local.target_role_name)].id
}
output "all_resources" {
value = [for r in null_resource.hi : r.id]
}
This outputs, for example
all_resources = [
"4350570701002192774",
"9173388682753384584",
"1634695740603384613",
"2098863759573339880",
]
target_resource = 1634695740603384613
So your example, I suppose, would look like
resource "aws_iam_role_policy_attachment" "polatt" {
role = aws_iam_role.role[index(var.somevariable, "TheRoleNameIWant")].id
policy_arn = "arn:aws:iam::aws:policy/..."
}
Update
Your comment below mentions that you actually have a more complicated data structure than just a list of names. I just wanted to mention that you can derive names from your JSON structure.
Assuming you have something like the following
variable "role_values" {
value = [
{
name = "foo",
other = "details",
fields = 3
},
{
name = "bar",
other = "yet more details",
fields = 3
}
]
}
you could derive just the names by using a local and the newer for loops TF 0.12 offers
locals {
role_names = [for role in var.role_values: role.name]
}
That way you don't have to store the names twice.