Terraform issue attaching a security group to my aws instances - count

Question

I'm new enough to terraform and think I am misunderstanding something with count and count.index usage.
I am creating some EC2 instances using the count parameter and it works fine
resource "aws_instance" "server" {
ami = data.aws_ami.app_ami.id
instance_type = "t2.micro"
key_name = "DeirdreKey"
subnet_id = aws_subnet.my_subnet_a.id
count = 2
tags = {
Name = "server.${count.index}"
}
I want to associated a security group with both instances so I created the below
resource "aws_network_interface_sg_attachment" "sg_attachment" {
security_group_id = aws_security_group.allow_internet.id
network_interface_id = aws_instance.server.primary_network_interface_id
}
However I am running into this error
Error: Missing resource instance key
on lb.tf line 57, in resource "aws_network_interface_sg_attachment" "sg_attachment":
57: network_interface_id = aws_instance.server.primary_network_interface_id
Because aws_instance.server has "count" set, its attributes must be
accessed on specific instances.
For example, to correlate with indices of a referring resource, use:
aws_instance.server[count.index]
I understand what the error is complaining about . Its because the local resource name I am referring to in not unique as I have created a count of 2 aws instances called "server". I dont know how to fix it though. I tried with the below
resource "aws_network_interface_sg_attachment" "sg_attachment" {
security_group_id = aws_security_group.allow_internet.id
network_interface_id = aws_instance.server[count.index].primary_network_interface_id
But then I get the below error
Error: Reference to "count" in non-counted context
on lb.tf line 53, in resource "aws_network_interface_sg_attachment" "sg_attachment":
53: network_interface_idaws_instance.server[count.index].primary_network_interface_id
The "count" object can only be used in "module", "resource", and "data"
blocks, and only when the "count" argument is set.
Does this mean I have to introduce the count.index into the local resource name? I tried it a few ways and it doesnt seem to work
resource "aws_instance" "server${count.index}" {

Answer 1

You need a count statement on the resource to use count.index. Count statements can get out of hand, so if you have multiple resources that logically need the same count, use a variable or local value:
local {
replications = 2
}
resource "aws_instance" "server" {
count = local.replications
ami = data.aws_ami.app_ami.id
instance_type = "t2.micro"
key_name = "DeirdreKey"
subnet_id = aws_subnet.my_subnet_a.id
tags = {
Name = "server.${count.index}"
}
}
resource "aws_network_interface_sg_attachment" "sg_attachment" {
count = local.replications
security_group_id = aws_security_group.allow_internet.id
network_interface_id = aws_instance.server[count.index].primary_network_interface_id
}
This creates one security group attachment per server, and gives you a list of servers you can reference as aws_instance.server[0] and aws_instance.server[1], and a list of attachments that you can reference in the same way.