Terraform conditional quantity with for_each - amazon-web-services

Question

I need an if statement with for_each.
If it is non-prod create resource in a single subnet, if production create resources in both subnets:
locals {
zone_data = {
a = {
subnet_id = data.aws_cloudformation_export.subnet1.value
}
b = {
subnet_id = data.aws_cloudformation_export.subnet2.value
}
}
}
module "batch_server" {
for_each = var.env_name == "non-prod" ? local.zone_data.a : local.zone_data
...
I get an error:
|----------------
| local.zone_data is object with 2 attributes
| local.zone_data.a is object with 1 attribute "subnet_id"
The true and false result expressions must have consistent types.
The given expressions are object and object, respectively.
I tried this:
for_each = var.env_name == "non-prod" ? [local.zone_data.a] : [local.zone_data]
and am getting similar error:
|----------------
| local.zone_data is object with 2 attributes
| local.zone_data.a is object with 1 attribute "subnet_id"
The true and false result expressions must have consistent types.
The given expressions are tuple and tuple, respectively.
Tried changing types but nothing seems to work

Answer 1

for_each expects a map with one element for each instance you want to declare. That means that if you want to declare only one instance then you need to produce a one-element map.
Here's one way to do that:
for_each = var.env_name == "non-prod" ? tomap({a = local.zone_data.a}) : local.zone_data
This can work because both arms of the conditional produce a map of objects. The true arm produces a single-element map while the second produces the whole original map.
Another option to avoid the inline map construction would be to factor out a per-environment lookup table into a separate local value:
locals {
all_zones = tomap({
a = {
subnet_id = data.aws_cloudformation_export.subnet1.value
}
b = {
subnet_id = data.aws_cloudformation_export.subnet2.value
}
})
env_zones = tomap({
non-prod = tomap({
for k, z in local.all_zones : k => z
if k == "a"
})
})
}
module "batch_server" {
for_each = try(local.env_zones[var.env_name], local.all_zones)
# ...
}
The try function call here means that local.all_zones is the fallback to use if var.env_name doesn't match one of the keys in local.env_zones.

Answer 2

How about a lookup with var.env_name as the key?
variable "env_name" {
type = string
default = "non_prod"
}
locals {
zone_data = {
prod = {
a = {
subnet_id = data.aws_cloudformation_export.subnet1.value
}
b = {
subnet_id = data.aws_cloudformation_export.subnet2.value
}
},
non_prod = {
a = {
subnet_id = data.aws_cloudformation_export.subnet1.value
}
}
}
}
module "batch_server" {
for_each = lookup(local.zone_data, var.env_name)
...