Demystifying nested loops in Terraform

main.tf

locals {
  # You have configuration in a data structure, a list of maps that defines an
  # instance of connections with the sources and the desitnations it is
  # supposed to connect to. However, it does not make sense to establish
  # a connection between 'the same endpoints' so we need to exclude those.
  data = [
    {
      name    = "foo"
      sources = ["a", "b",]
      dests   = ["x", "y", "a",]
    },
    {
      name    = "bar"
      sources = ["a", "b",]
      dests   = ["x", "b",]
    },

  ]

  # We would need nested loops to come up with all the connection objects.
  # However, this cannot be achieved with for_each in the connection resource
  # type, because you cannot nest there. In such a scenario, it makes sense to
  # build the final data structure which is a group of 'atomic' data needed for
  # each connection.
  res = merge(flatten([
    for o in local.data :
    [
      for s in o.sources :
      {
        for d in o.dests :
        "${o.name}_${s}_${d}" => {
          source : s
          dest : d
        } if s != d
      }
    ]
  ])...)

  # In the first two loops, we make a list of lists. This is just for the sake
  # of iterating. At the bottom level, we build a map of objects. This is the
  # meat of it. Here we build the final data set needed for a single
  # connection.  The map key is designed to be unique across elments. It's also
  # best to make it descriptive, so when you see it in the state, it makes
  # sense to you!
  #
  # We exclude the case where source and destination are the same with
  # `if s != d`
  #
  # At the top level, we flatten the list of lists we built just for the sake
  # of iterating and then we use ellipsis(...) on the resulting list. This is
  # to dynamically split the list of maps into a bunch of map elements so that
  # we can merge it all into one big map.
}

output "res" {
  value = local.res
}


resource "local_file" "main" {
  for_each = local.res
  content  = jsonencode(each.value)
  filename = "${path.module}/data/${each.key}"
}