Terraform: How to use a variable inside of another variable?

Question

I'm trying to design it so that all configurable parameters exist within variables.tf...the issue I'm running into currently is for provisioning EC2 instances. I'd like to be able to define the security group to use, but the input variable only accepts strings, so I wasn't able to reference the security group id.

I moved on to trying this, which is looking up the resource id dynamically by inserting the name (look at security_groups_name and subnet_name:

variable "ansible_ami" { 
    type =  "map"
    default = {
            name = "ansible"
            ami = "ami-4836a428"
            size = "t2.micro"
            ebs_size = "8"
            ebs_type = "gp2" #general purpose ssd
            enable_public_ip = true
            security_groups_name = "JumpBox"
            subnet_name = "pubnet1"
    }
}


resource "aws_instance" "ansible" {
    ami = "${var.ansible_ami["ami"]}"
    instance_type = "${var.ansible_ami["size"]}"
    security_groups = "${aws_security_group.${var.ansible_ami["security_groups_name"]}.id}"
    subnet_id = "${aws_subnet.${var.ansible_ami["subnet_name"]}.id}"
    associate_public_ip_address = "${var.ansible_ami["enable_public_ip"]}"
    #user_data = ""

    ebs_block_device {
        device_name = "/dev/sdb"
        volume_size = "${var.ansible_ami["ebs_size"]}"
        volume_type = "${var.ansible_ami["ebs_type"]}"
    }

    tags {
        Name = "${var.ansible_ami["name"]}"
    }
}

However, that is not working either since i'm getting syntax errors from the lines where I'm trying to next variables. I'm looking for ideas or direction on how to accomplish this.

Answer 1

Largely inspired by the documentation the idea is to create a terraform datasource from a filter of the available resources and then use this as entry point in your ami resource:

variable "security_group_id" {}
variable "subnet_id" {}

data "aws_security_group" "selected" {
  filter {
    name = "${var.ansible_ami["security_groups_name"]}}"
  }
}

data "aws_subnet" "selected" {
  filter {
    name = "${var.ansible_ami["subnet_name"]}"
  }
}

resource "aws_instance" "ansible" {
    ami = "${var.ansible_ami["ami"]}"
    instance_type = "${var.ansible_ami["size"]}"
    security_groups = "${data.aws_security_group.selected.id}"
    subnet_id = "${data.aws_subnet.selected.id}"
    associate_public_ip_address = "${var.ansible_ami["enable_public_ip"]}"
    #user_data = ""

I'm not using terraform myself, so this could be plain wrong.