Terraform: aws_subnet_ids is not iterable

[nfplatzke]

Terraform Version

Terraform v0.12.0
+ provider.aws v2.12.0

Terraform Configuration Files

data "aws_subnet_ids" "example" {
  vpc_id = "${var.vpc_id}"
}

data "aws_subnet" "example" {
  count = "${length(data.aws_subnet_ids.example.ids)}"
  id    = "${data.aws_subnet_ids.example.ids[count.index]}"
}

output "subnet_cidr_blocks" {
  value = ["${data.aws_subnet.example.*.cidr_block}"]
}

Debug Output

Crash Output

Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 9
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.


Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 0
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.


Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 1
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

2019/05/30 16:43:17 [TRACE] statemgr.Filesystem: removing lock metadata file .terraform.tfstate.lock.info
This value does not have any indices.


Error: Invalid index

2019/05/30 16:43:17 [TRACE] statemgr.Filesystem: unlocking terraform.tfstate using fcntl flock
  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 8
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.


Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 5
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.


Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 7
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.


Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 6
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.


Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
2019-05-30T16:43:17.563-0400 [DEBUG] plugin: plugin process exited: path=.terraform/plugins/darwin_amd64/terraform-provider-aws_v2.12.0_x4 pid=70561
2019-05-30T16:43:17.563-0400 [DEBUG] plugin: plugin exited
    |----------------
    | count.index is 3
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.


Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 2
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.


Error: Invalid index

  on vpc2.tf line 7, in data "aws_subnet" "example":
   7:   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
    |----------------
    | count.index is 4
    | data.aws_subnet_ids.example.ids is set of string with 10 elements

This value does not have any indices.

Expected Behavior

I should be able to loop through a set of subnets returned by aws_subnet_ids. I encountered this error in my own code while upgrading from 0.11 to 0.12 but was able to replicate it using the exact example code on the aws_subnet_ids page (https://www.terraform.io/docs/providers/aws/d/subnet_ids.html)

Actual Behavior

It crashes saying that the object has no indices. So either some behavior changed or the documentation is wrong. I have tried everything I can think of to fix this but nothing works.

Steps to Reproduce

terraform init

terraform apply

Additional Context

References

[mildwonkey]

Hi @nfplatzke !
I'm sorry for this confusing behavior!
As the error states, data.aws_subnet_ids.example.ids is set, not a list. A set is _iterable_ but not _indexable_, and in this instance the fastest workaround is to convert the set to a list with the tolist() function:

data "aws_subnet" "example" {
  count = length(data.aws_subnet_ids.example.ids)
  id    = tolist(data.aws_subnet_ids.example.ids)[count.index]
}

[nfplatzke]

The documentation and example code for aws_subnet_ids should be updated to reflect this.

[mildwonkey]

That's a good point @nfplatzke. Can you open a new issue in the aws provider repository with this request? The provider documentation is in the provider's repository.

[goodspark]

I just ran into this. Would it be possible to revert this change to become a list again?

tolist won't work in some cases because it does not sort the entries of a set deterministically. Thus, calling this multiple times in a template may yield non-deterministic results.

https://www.terraform.io/docs/configuration/functions/tolist.html

Since set elements are not ordered, the resulting list will have an undefined order that will be consistent within a particular run of Terraform

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