Describes how Terraform describes vpc_id when passing it to subnet resources.

Prerequisites/What you want to achieve

I would like to create a simple configuration of VPC and Subnet in the AWS environment with Terraform.
It seems that the vpc_id created by VPC is not passed well to Subnet, so
I would appreciate it if you could give me some advice on how to solve this problem.

For configuration, assume multiple resources are created. We are calling using for_each and lookup.

Problems/Error Messages you are experiencing

$terraform plan-var-file=prd/resource-prd-vars.tfvars 
module.create_vpc_module.aws_vpc.vpc ["vpc01"]—Refreshing state... [id=vpc-0419f19a0980c568f]

Error: Invalid index

  on..\modules\vpc\vpc-main.tf line 15, in resource "aws_subnet" "subnet":
  15: vpc_id =aws_vpc.vpc [each.key].id
    |----------------
    | aws_vpc.vpc is object with 1 attribute "vpc01"
    | each.key is "subnet01"

The given key does not identify an element in this collection value.

■ directory configuration

V - VPC
    --modules
    │  v-vpc
    │          │          output.tf
    │          │          vpc-main.tf
    │          │          vpc-variavles.tf
    │
    --resource
        │  │  backend.tf
        │  │  provider.tf
        │  │  resource-main.tf
        │  │  resource-variables.tf
        │  terraform.tfstate
        │  terraform.tfstate.backup
        --prd
                resource-prd-vars.tfvars

■modules/vpc/vpc-main.tf

#-----------------------------------------------
# VPC
# ---------------------------------------------
resource "aws_vpc" "vpc" {
  for_each=var.vpc
  cidr_block=lookup(each.value, "cidr_block", null)
  tags = lookup (each.value, "tags", null)
}

# ---------------------------------------------
# Subnet
# ---------------------------------------------
resource "aws_subnet" "subnet" {
  for_each=var.subnet
  vpc_id =aws_vpc.vpc [each.key].id
  availability_zone=lookup(each.value, "availability_zone", null)
  cidr_block=lookup(each.value, "cidr_block", null)
  map_public_ip_on_launch=lookup(each.value, "map_public_ip_on_launch", null)
  tags = lookup (each.value, "tags", null)
}

■modules/vpc/vpc-variables.tf

#-----------------------------------------------
# Variables Used When Creating VPCs
# ---------------------------------------------

variable "vpc" {
  default={}
}

# ---------------------------------------------
# Variables Used When Creating Subnet
# ---------------------------------------------

variable "subnet" {
  default={}
}

■ resource /resource-main.tf

#-----------------------------------------------
# Invoking Various Modules
# ---------------------------------------------
module "create_vpc_module" {
  source="../modules/vpc"
  vpc = var.vpc
  subnet=var.subnet
}

■ resource /resource-variables.tf

#-----------------------------------------------
# Invoking Variables
# ---------------------------------------------
variable "vpc" {}

variable "subnet" {}

■ resource /prd /resource-prd-vars.tfvars

#vpctfvars
# actual variable value declaration

# ---------------------------------------------
# VPC
# ---------------------------------------------

vpc = {
  vpc01 = {
    description="vpc for testing"
    cidr_block="10.0.0.0/16"
    tags = {
      "Name": "prd-vpc",
      "Env": "prd"
    }
  }
}

subnet = {
  subnet01 = {
    availability_zone="ap-northeast-1a"
    cidr_block="10.0.0.0/20"
    map_public_ip_on_launch="true"
    tags = {
      "Name": "subnet",
      "Env": "prd"
    }
  }
}

Tried

Use output to pass values to module, data format, etc. I tried, but it doesn't seem to have been received properly.

Supplementary information (for example, FW/Tool Version)

terraform_version": "0.14.6"

aws terraform

2022-09-29 22:30

1 Answers

 resource "aws_subnet" "subnet" {
  for_each=var.subnet
  vpc_id =aws_vpc.vpc [each.key].id

The variable subnet key is subnet01, so each.key returns subnet01.

Therefore, vpc_id=aws_vpc.vpc["subnet01"].id but vpc_id=aws_vpc.vpc["vpc01"].id

I don't know if it fits the questioner's design intentions, but if you modify .tfvars and match the key of the variable vpc with the subnet, it should work (not verified).

vpc={
  01 = {
  ...
  }
}

subnet = {
  01 = {
  ...
  }
}

2022-09-29 22:30

If you have any answers or tips

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