map list of maps to a list of selected field values in terraform

Question

If resources use a count parameter to specify multi resources in terraform there is a simple syntax for providing a list/array of dedicated fields for the resource instances.

for example

aws_subnet.foo.*.id

Since quite a number of versions it is possible to declare variables with a complex structure, for example lists of maps.

variable "data" {
  type = "list"
  default = [
    {
      id = "1"
      ...
    },
    {
      id = "10"
      ...
    }
  ]
}

I'm looking for a possibility to do the same for varaibles I can do for multi resources: a projection of an array to an array of field values of the array elements.

Unfortunately

var.data.*.id

does not work as for resources. Is there any possibility to do this?

Answer 1

NOTE: This answer and its associated question are very old at this point, and this answer is now totally stale. I'm leaving it here for historical reference, but nothing here is true of modern Terraform.

---

At the time of writing, Terraform doesn't have a generalized projection feature in its interpolation language. The "splat syntax" is implemented as a special case for resources.

While deep structure is possible, it is not yet convenient to use, so it's recommended to still keep things relatively flat. In future it is likely that new language features will be added to make this sort of thing more usable.

Answer 2

A potentially simpler answer is to use the zipmap function.

Starting with an environment variable map compatible with ECS template definitions:

locals {
  shared_env = [
    {
      name  = "DB_CHECK_NAME"
      value = "postgres"
    },
    {
      name  = "DB_CONNECT_TIMEOUT"
      value = "5"
    },
    {
      name  = "DB_DOCKER_HOST_PORT"
      value = "35432"
    },
    {
      name  = "DB_DOCKER_HOST"
      value = "localhost"
    },
    {
      name  = "DB_HOST"
      value = "my-db-host"
    },
    {
      name  = "DB_NAME"
      value = "my-db-name"
    },
    {
      name  = "DB_PASSWORD"
      value = "XXXXXXXX"
    },
    {
      name  = "DB_PORT"
      value = "5432"
    },
    {
      name  = "DB_QUERY_TIMEOUT"
      value = "30"
    },
    {
      name  = "DB_UPGRADE_TIMEOUT"
      value = "300"
    },
    {
      name  = "DB_USER"
      value = "root"
    },
    {
      name  = "REDIS_DOCKER_HOST_PORT"
      value = "6380"
    },
    {
      name  = "REDIS_HOST"
      value = "my-redis"
    },
    {
      name  = "REDIS_PORT"
      value = "6379"
    },
    {
      name  = "SCHEMA_SCRIPTS_PATH"
      value = "db-scripts"
    },
    {
      name  = "USE_LOCAL"
      value = "false"
    }
  ]
}

In the same folder launch terraform console for testing built-in functions. You may need to terraform init if you haven't already.

terraform console

Inside the console type:

zipmap([for m in local.shared_env: m.name], [for m in local.shared_env: m.value])

Observe the output of each list-item-map being a name-value-pair of a single map:

{
  "DB_CHECK_NAME" = "postgres"
  "DB_CONNECT_TIMEOUT" = "5"
  "DB_DOCKER_HOST" = "localhost"
  "DB_DOCKER_HOST_PORT" = "35432"
  "DB_HOST" = "my-db-host"
  "DB_NAME" = "my-db-name"
  "DB_PASSWORD" = "XXXXXXXX"
  "DB_PORT" = "5432"
  "DB_QUERY_TIMEOUT" = "30"
  "DB_UPGRADE_TIMEOUT" = "300"
  "DB_USER" = "root"
  "REDIS_DOCKER_HOST_PORT" = "6380"
  "REDIS_HOST" = "my-redis"
  "REDIS_PORT" = "6379"
  "SCHEMA_SCRIPTS_PATH" = "db-scripts"
  "USE_LOCAL" = "false"
}

Answer 3

UPDATE

Massive fancy features have been added into terraform since Terraform 0.12 was released, e.g., list comprehension, with which the solution is super easy.

locals {
  ids = [for d in var.data: d.id]
  #ids = [for d in var.data: d["id"]]  #same
}

# Then you could get the elements this way,
#     local.ids[0]

---

Solution before terraform 0.12

template_file can help you out.

data "template_file" "data_id" {
  count = "${length(var.data)}"
  template = "${lookup(var.data[count.index], "id")}"
}

Then you get a list "${data.template_file.data_id.*.rendered}", whose elements are value of "id".

You can get its element by index like this

"${data.template_file.data_id.*.rendered[0]}"

or through function element()

"${element(data.template_file.data_id.*.rendered, 0)}"

Answer 4

If have found a working solution using template rendering to by-pass the list of map's issue:

resource "aws_instance" "k8s_master" {
  count                       = "${var.master_count}"
  ami                         = "${var.ami}"
  instance_type               = "${var.instance_type}"
  vpc_security_group_ids      = ["${aws_security_group.k8s_sg.id}"]
  associate_public_ip_address = false
  subnet_id                   = "${element(var.subnet_ids,count.index % length(var.subnet_ids))}"
  user_data                   = "${file("${path.root}/files/user_data.sh")}"
  iam_instance_profile        = "${aws_iam_instance_profile.master_profile.name}"

  tags = "${merge(
    local.k8s_tags,
    map(
      "Name", "k8s-master-${count.index}",
      "Environment", "${var.environment}"
    )
  )}"
}

data "template_file" "k8s_master_names" {
  count    = "${var.master_count}"
  template = "${lookup(aws_instance.k8s_master.*.tags[count.index], "Name")}"
}

output "k8s_master_name" {
  value = [
    "${data.template_file.k8s_master_names.*.rendered}",
  ]
}

This will result in the following output:

k8s_master_name = [
    k8s-master-0,
    k8s-master-1,
    k8s-master-2
]