fix: Add security group name

main.tf

@@ -1,6 +1,7 @@
 locals {
   port             = "${var.port == "" ? "${var.engine == "aurora-postgresql" ? "5432" : "3306"}" : var.port}"
   master_password  = "${var.password == "" ? random_id.master_password.b64 : var.password}"
+  db_subnet_group_name = "${var.db_subnet_group_name == "" ? join("", aws_db_subnet_group.this.*.name) : var.db_subnet_group_name}"
   backtrack_window = "${var.backtrack_window == "" ? "${var.engine == "aurora" ? "0" : ""}" : var.backtrack_window}"
 }
 
@@ -10,6 +11,8 @@ resource "random_id" "master_password" {
 }
 
 resource "aws_db_subnet_group" "this" {
+  count = "${var.db_subnet_group_name == "" ? 1 : 0}"
+
   name        = "${var.name}"
   description = "For Aurora cluster ${var.name}"
   subnet_ids  = ["${var.subnets}"]
@@ -36,7 +39,7 @@ resource "aws_rds_cluster" "this" {
   preferred_backup_window             = "${var.preferred_backup_window}"
   preferred_maintenance_window        = "${var.preferred_maintenance_window}"
   port                                = "${local.port}"
-  db_subnet_group_name                = "${aws_db_subnet_group.this.name}"
+  db_subnet_group_name                = "${local.db_subnet_group_name}"
   vpc_security_group_ids              = ["${concat(list(aws_security_group.this.id), var.vpc_security_group_ids)}"]
   scaling_configuration               = "${var.scaling_configuration}"
   snapshot_identifier                 = "${var.snapshot_identifier}"
@@ -60,7 +63,7 @@ resource "aws_rds_cluster_instance" "this" {
   engine_version                  = "${var.engine_version}"
   instance_class                  = "${var.instance_type}"
   publicly_accessible             = "${var.publicly_accessible}"
-  db_subnet_group_name            = "${aws_db_subnet_group.this.name}"
+  db_subnet_group_name            = "${local.db_subnet_group_name}"
   db_parameter_group_name         = "${var.db_parameter_group_name}"
   preferred_maintenance_window    = "${var.preferred_maintenance_window}"
   apply_immediately               = "${var.apply_immediately}"

variables.tf

@@ -235,6 +235,11 @@ variable "vpc_security_group_ids" {
   default     = []
 }
 
+variable "db_subnet_group_name" {
+  description = "The existing subnet group name to use"
+  default     = ""
+}
+
 variable "backtrack_window" {
   description = "The target backtrack window, in seconds. Only available for aurora engine currently. To disable backtracking, set this value to 0. Defaults to 0. Must be between 0 and 259200 (72 hours)"
   default     = "0"