Guarantee unique keys in Terraform

When using Terraform to dynamically create resources based on lists of maps you probaby have run into this issue. Consider the following list of maps that represents IAM access on a generic cloud resource:

locals = {
    members = [
        {
            member   = "[email protected]"
            resource = "projects/12345"
            role     = "roles/owner"
        },
        {
            member   = "[email protected]"
            resource = "projects/12345"
            role     = "roles/reader"
        }
    ]
}

If we want to iterate over this list to create a dynamic amount of resources (using for_each) we need to convert it to a map. However there is no way you can construct an map key from any of the separate attributes and guarantee uniqueness. So how can we solve this?

The trick is to create a combination of all the attributes. But simply concatenating all the attributes in a string results in very long keys. To solve this, and get predictable key lengths, we can use an md5 hash:

locals = {
    unique_members = { for key, member in local.members : md5("${member.member}/${member.resource}/${member.role}") => member }
}

This results in the following data:

{
    "0f334c4500b1faab57203343199d5c86" = {
        member   = "[email protected]"
        resource = "projects/12345"
        role     = "roles/owner"
    },
    "c02f629ef8bf2b413a203c4dcafa60c1" = {
        member   = "[email protected]"
        resource = "projects/12345"
        role     = "roles/reader"
    }
}

Now we can use it in our for_each iterator:

resource "iam_member" "default" {
    for_each = local.unique_members

    member   = each.value.member
    resource = each.value.resource
    role     = each.value.role
}

Now you know a simple trick to convert a list of maps into an iterable map with unique keys.

As a bonus you now get alerted about duplicate list entries, as they would result in dulicate map keys causing Terraform to throw an error!