Terraform use variable based on another value passed during runtime

Question

During runtime I want to tell terraform which of those variables are to be used from my .tfvars file I have the following values:

website-review-author                  = "test1/fwsfds/34131"
website-dev-publish                 = "test2/fwsfds/34131"
website-other-dispatcher              = "test3/fwsfds/34131"

So I want to data_source_id to use those variables based on a parameter during runtime:

resource "azurerm_managed_disk" "datadisk" {
  name                 = "${var.default_prefix}-${var.environment}-datadisk"
  location             = "${var.default_region}"
  resource_group_name  = "${var.permanent_resources_rg}"
  storage_account_type = "${var.disk_type}"
  create_option        = "Copy"
  source_resource_id   = "${var.website_review_publish.id}" ## I need  to be a dynamic variable. So sometimes it will se website_review_author, other time website-dev-publish
  disk_size_gb         = "${var.data_disk_size_gb}"

  tags = "${var.tags}"

  depends_on = [
    azurerm_storage_account.storage
    #azurerm_managed_disk.osdisk
  ]
}

Is it possible in terrraform to switch between variables, so it gets the proper value. The condition is :

 If (var.environment == website-review-author) ## Can be
 website-dev-publish or #website-other-dispathcer 
  then 
 source_resource_id = var.website-review-author #or other value based
 on the parameter

I'm new to terraform, so still learning my ways around. I need something like if condition.

Answer 1

You could use the ternary operator [1]. If you decide to go down that route, the code would then look like:

resource "azurerm_managed_disk" "datadisk" {
  name                 = "${var.default_prefix}-${var.environment}-datadisk"
  location             = "${var.default_region}"
  resource_group_name  = "${var.permanent_resources_rg}"
  storage_account_type = "${var.disk_type}"
  create_option        = "Copy"
  source_resource_id   = var.environment == website-review-author ? var.website-review-author : var.environment == website-dev-publish ? var.website-dev-publish : var.environment == website-other-dispatcher ? var.website-other-dispatcher : null 
  disk_size_gb         = "${var.data_disk_size_gb}"

  tags = "${var.tags}"

  depends_on = [
    azurerm_storage_account.storage
  ]
}

It could be made more complex by using other logical operators [2] (e.g., or), however if you need all three different values, then I suggest using workspaces [3]. For example, in that case you could use the workspace name with the ternary operator to decide which value will be assigned:

source_resource_id = terraform.workspace == <someworkspacename> ? var.webiste-review-author : var.website-dev-publish

EDIT: I added all three conditions. It is really ugly but it should work. I suggest creating a different variable of type map and then using it for creating resources. Additionally, whenever the value for environment is changed, the resource will probably be destroyed and recreated, hence why it is better to use count or for_each.

---

[1] https://www.terraform.io/language/expressions/conditionals

[2] https://www.terraform.io/language/expressions/operators#arithmetic-and-logical-operators

[3] https://www.terraform.io/cli/workspaces#the-purpose-of-workspaces