Produce an output from a module that using 'this' keyword

I'm trying to produce an output from a terraform child module which creates a number of subnets based on a variable (which is a map of objects). I create subnets with:

resource "aws_subnet" "this" {
  for_each = var.subnets

  vpc_id                  = aws_vpc.vpc.id
  availability_zone       = each.value.az
  cidr_block              = each.value.cidr_block
  map_public_ip_on_launch = each.value.public

  tags = {
    Name = each.key
    Type = each.value.public ? "public" : "private"
  }
}

output.tf in the same folder as the module:

output "vpc_subnets_id" {
  description = "The CIDR block of the VPC"
  value       = values(aws_subnet.this)[*].id
}

output.tf in the main module directory (where main.tf is):

output "vpc_subnets_ids" {
  description = "The CIDR block of the VPC"
  value       = values(module.vpc.aws_subnet.this)[*].id
}

This produces an error:

on output.tf line 17, in output "vpc_subnets_ids":
│   17:   value       = values(module.vpc.aws_subnet.this)[*].id
│     ├────────────────
│     │ module.vpc is a object
│ 
│ This object does not have an attribute named "aws_subnet".

Why doesn't it work, and how to make it work?

Your module output is named vpc_subnets_id, and not aws_subnet. It also seems as if your structure query and member accessing is redundant across the two output. The query and accessing could also potentially fail for an empty var.subnets, and potentially be unreliable. These fixes all combine as:

# declared module output
output "vpc_subnets_id" {
  description = "The CIDR block of the VPC"
  value       = [for subnet in aws_subnet.this : subnet.id]
}

# root module output
output "vpc_subnets_ids" {
  description = "The CIDR block of the VPC"
  value       = module.vpc.vpc_subnets_id
}