Invalid for_each argument during the aws_lb_target_group_attachment

Terraform
3

Okay, I have figure out in my way, which is working fine for now. In my ec2 child-module, I have a data-source like this:

ec2/data.tf

// List of instance attributes by role
data "aws_instance" "by_role" {
  for_each = {
    for ic in range(var.inst_count): "${var.inst_role}0${ic+1}" => ic
  }
  instance_tags  = {
    Name = "${var.vpc_names[var.idx]}${each.key}"
  }
  instance_id    = aws_instance.inst[substr(each.key,4,2)-1].id
}
// 
output "inst_by_role" {
  value = data.aws_instance.by_role
}

That returns a map like this:

inst_info = {
  "asa01" = {
    "ami" = "ami-06a5087891b6d8eb8"
    "arn" = "arn:aws:ec2:us-east-1:619xx:instance/i-0939b2db7xx"
    "associate_public_ip_address" = false
    ..........
  }
  "asa02" = {
    "ami" = "ami-06a5087891b6d8eb8"
    "arn" = "arn:aws:ec2:us-east-1:619xx:instance/i-0ab3f8dae7xx"
    "associate_public_ip_address" = false
    .........
  }
}

and in the nlb child-module:

nlb/listener.tf

variable "inst_info" {
  #type       = map
  description = "List of attributes of the NLB members"
}
// Attach the target groups to the instance(s)
resource "aws_lb_target_group_attachment" "tgr_attachment" {
  for_each = {
    for pair in setproduct(keys(aws_lb_target_group.nlb_target_groups), keys(var.inst_info)):
    "${pair[0]}:${pair[1]}" => {
      target_group = aws_lb_target_group.nlb_target_groups[pair[0]]
      inst_name    = var.inst_info[pair[1]]
    }
  }
  target_group_arn = each.value.target_group.arn
  target_id        = each.value.inst_name.id
  port             = each.value.target_group.port
}

then feed the data in the root-module:

module "asa-nlb" {
  source    = "../../modules/nlb"
  inst_role = "asa"
  inst_info = module.dfw.inst_by_role
  .......
}

this reply from @apparentlymart helped me a lot to look for a alternative way to get it working.

-San