Only set monitoring_role_arn if monitoring_interval > 0

README.md

@@ -4,7 +4,7 @@ A Terraform module to create an Amazon Web Services (AWS) PostgreSQL Relational
 
 ## Usage
 
-```javascript
+```hcl
 module "postgresql_rds" {
   source = "github.com/azavea/terraform-aws-postgresql-rds"
   vpc_id = "vpc-20f74844"
@@ -40,6 +40,26 @@ module "postgresql_rds" {
 }
 ```
 
+### Note about Enhanced Monitoring Support
+
+If the `monitoring_interval` passed as an input to this module is `0`, an empty `monitoring_role_arn` value will be threaded into the `aws_db_instance` resource. 
+
+This is because, if a value for `monitoring_role_arn` is threaded into an `aws_db_instance`, along with a `monitoring_interval` of `0`, the following error will occur:
+
+```bash
+InvalidParameterCombination: You must specify a MonitoringInterval value other than 0 when you specify a MonitoringRoleARN value.
+```
+
+This behavior doesn't match the information in the [Terraform docs](https://www.terraform.io/docs/providers/aws/r/db_instance.html#monitoring_interval) for the resource. Both the `monitoring_role_arn` and `monitoring_interval` arguments to `aws_db_instance` should be optional, and setting a `monitoring_interval` to `0` should be enough to disable enhanced monitoring, irrespective of the value of `monitoring_role_arn`. 
+
+See the following open issues for the Terraform AWS provider:
+
+https://github.com/terraform-providers/terraform-provider-aws/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+MonitoringInterval+
+
+See a good explaination of what's causing this here:
+
+https://github.com/terraform-providers/terraform-provider-aws/issues/315#issuecomment-437404518
+
 ## Variables
 
 - `vpc_id` - ID of VPC meant to house database
@@ -92,4 +112,3 @@ module "postgresql_rds" {
 - `endpoint` - Public DNS name and port separated by a `:`
 - `hosted_zone_id` - The zone id for the autogenerated DNS name given in `endpoint`. 
    Use this when creating a short-name DNS [alias](https://www.terraform.io/docs/providers/aws/r/route53_record.html#alias-record) for the `endpoint` 
-

main.tf

@@ -73,7 +73,7 @@ resource "aws_db_instance" "postgresql" {
   parameter_group_name       = "${var.parameter_group}"
   storage_encrypted          = "${var.storage_encrypted}"
   monitoring_interval        = "${var.monitoring_interval}"
-  monitoring_role_arn        = "${aws_iam_role.enhanced_monitoring.arn}"
+  monitoring_role_arn        = "${var.monitoring_interval > 0 ? aws_iam_role.enhanced_monitoring.arn : ""}"
 
   tags {
     Name        = "DatabaseServer"