Transform an object (map) into a distinct list with terraform

Question

With Terraform, is it possible to convert an object something like

locals {
  data = {
    "project1" = {
      user_assigned           = ["user1", "user2", "user3"]
     }
    "project2" = {
      user_assigned           = ["user2", "user3", "user4"]
     }
  }

to an output like

user1 = ["project1"]
user2 = ["project1","project2"]
user3 = ["project1","project2"]
user4 = ["project2"]

Note that data is an object with maps of keys(projects) and values(user_assigned)

Answer 1

You can't dynamically create fully independent variables. Instead you can create a map in few ways. One way would be with the help of transpose and zipmap:

output "test1" {
    value = transpose(zipmap(keys(local.data), values(local.data)[*].user_assigned))
}

Resulting in:

test1 = tomap({
  "user1" = tolist([
    "project1",
  ])
  "user2" = tolist([
    "project1",
    "project2",
  ])
  "user3" = tolist([
    "project1",
    "project2",
  ])
  "user4" = tolist([
    "project2",
  ])
})

Answer 2

Here's another way to do it, which I'm sharing just in case it's interesting -- the accepted answer is a fine approach too.

locals {
  data = {
    "project1" = {
      user_assigned = ["user1", "user2", "user3"]
    }
    "project2" = {
      user_assigned = ["user2", "user3", "user4"]
    }
  }

  project_user = flatten([
    for proj_name, proj in local.data : [
      for username in proj.user_assigned : {
        project_name = proj_name,
        username     = username
      }
    ]
  ])
}

output "example" {
  value = {
    for pu in local.project_user :
    pu.username => pu.project_name...
  }
}

Outputs:

example = {
  "user1" = [
    "project1",
  ]
  "user2" = [
    "project1",
    "project2",
  ]
  "user3" = [
    "project1",
    "project2",
  ]
  "user4" = [
    "project2",
  ]
}

---

I typically use this sort of approach because a data structure like that intermediate local.project_user value -- which is a list with an element for each project/user pair -- often ends up being useful when declaring resources that represent those pairings.

There wasn't any context in the question about what these projects and users represent or which provider they might related to, so I'm going to use github_team and github_team_membership as an example to illustrate what I mean:

resource "github_team" "example" {
  for_each = local.data

  name = each.key
}

resource "github_team_membership" "example" {
  for_each = {
    for pu in local.project_user : "${pu.username}:${pu.project_name}" => pu
  }

  team_id  = github_team.example[each.value.project_name].id
  username = each.value.username
}

Lots of providers have resources that represent a relationship between two objects like this, and so having an intermediate data structure that contains an element for each pair is a useful building block for those cases, and then you can derive from that mappings in either direction as I did in the output "example" in my original snippet.