Terraform 0.12 nested for loops

Question

I am trying to implement nested for loops using Terraform 0.12's new features in order to loop through AWS IAM users, each of which can have one or more policies attached. The variable used to represent this list is of type map(list(string)) and looks something like this:

{
  "user 1" = [ "policy1", "policy2" ],
  "user 2" = [ "policy1" ]
}

Getting the list of users to create is easy enough via keys(), but since there is currently no mechanism for nesting looped resource creation in Terraform, the policy attachments have to happen as a singular loop independent of each user. So, I am attempting to construct a list of user:policy associations from the map input that would look something like this based on the example above:

[
  [ "user1", "policy1" ],
  [ "user1", "policy2" ],
  [ "user2", "policy1" ]
]

I am attempting construct that list and store it in a local variable like so, where var.iam-user-policy-map is the input map:

locals {
  ...
  association-list = [
    for user in keys(var.iam-user-policy-map):
    [
      for policy in var.iam-user-policy-map[user]:
      [user, policy]
    ]
  ]
  ...
}

However, I am getting errors when attempting to access the values in that nested list. I am trying to access the user portion of the association with the reference local.association-list[count.index][0] and the policy with local.association-list[count.index][1], but on running terraform plan it errors out:

Error: Incorrect attribute value type

  on main.tf line 27, in resource "aws_iam_user_policy_attachment" "test-attach":
  27:   user = local.association-list[count.index][0]

Inappropriate value for attribute "user": string required.


Error: Incorrect attribute value type

  on main.tf line 27, in resource "aws_iam_user_policy_attachment" "test-attach":
  27:   user = local.association-list[count.index][0]

Inappropriate value for attribute "user": string required.


Error: Invalid index

  on main.tf line 28, in resource "aws_iam_user_policy_attachment" "test-attach":
  28:   policy_arn = "arn:aws-us-gov:iam::aws:policy/${local.association-list[count.index][1]}"
    |----------------
    | count.index is 0
    | local.association-list is tuple with 2 elements

The given key does not identify an element in this collection value.


Error: Invalid template interpolation value

  on main.tf line 28, in resource "aws_iam_user_policy_attachment" "test-attach":
  28:   policy_arn = "arn:aws-us-gov:iam::aws:policy/${local.association-list[count.index][1]}"
    |----------------
    | count.index is 1
    | local.association-list is tuple with 2 elements

Cannot include the given value in a string template: string required.

What am I doing wrong?

Answer 1

The for expression in your local value association-list is producing a list of list of lists of strings, but your references to it are treating it as a list of lists of strings.

To get the flattened representation you wanted, you can use the flatten function, but because it would otherwise group everything into a single flat list I'd recommend making the innermost value an object instead. (That will also make the references to it clearer.)

locals {
  association-list = flatten([
    for user in keys(var.iam-user-policy-map) : [
      for policy in var.iam-user-policy-map[user] : {
        user   = user
        policy = policy
      }
    ]
  ])
}

The result of this expression will have the following shape:

[
  { user = "user1", policy = "policy1" },
  { user = "user1", policy = "policy2" },
  { user = "user2", policy = "policy2" },
]

Your references to it can then be in the following form:

user = local.association-list[count.index].user

policy_arn = "arn:aws-us-gov:iam::aws:policy/${local.association-list[count.index].policy}"