How to create a set by loop over a nested map - terraform

Question

I have a map that I want to read in locals and generate a new map from. One field in the new map will be a set containing the values from the nested data structure. I can't figure out the syntax to do this.
//I want to generate a set of all zones from the nested zone fields
variable "my_var" {
type = object({
name = string
google_bigtable_clusters = any
})
default = {
app_name = "sdfsdfds"
instances = {
instance01 = [
{
zone = "asia-east1-a"
num_nodes = 1
},
{
zone = "asia-east1-b"
num_nodes = 1
},
{
zone = "asia-east1-c"
num_nodes = 1
},
{
zone = "asia-east2-a"
num_nodes = 1
},
],
instance02 = [
{
zone = "europe-west2-a"
num_nodes = 1
},
{
zone = "europe-west2-b"
num_nodes = 1
},
{
zone = "europe-west2-c"
num_nodes = 1
},
{
zone = "europe-west3-a"
num_nodes = 1
},
]
}
}
}
This throws The key expression produced an invalid result: string required.
// locals
new_map = {
some_field = "arbitrary string"
set_of_zones = {
for item in var.my_var.instances : item => {
for subitem in item : subitem.zone => {
zone = subitem.zone
}
}
}
}
I also tried to get the key name but that didn't work: for item in var.my_var.instances : item.key => {
Edit
I was able to do this but I don't understand why I don't have access to the key name here. I want to use the instance01, instance02, etc key name here: for item in var.my_var.instances : item[0].zone => {.

Answer 1

First, your type for your variable is all messed up. You have:
type = object({
name = string
google_bigtable_clusters = any
})
This means that Terraform will accept a value for the variable only if it has these two fields: name (a string) and google_bigtable_clusters (can be anything).
Your default value has neither of these fields. Instead, it only contains app_name and instances, so that's likely the cause of the first issue.
Regarding why you can't access the key name in your for loop, you need to specify both the key and value:
set_of_zones = {
for key, val in var.my_var.instances :
key => {
for subval in val:
subval.zone => {
zone = subval.zone
}
}
}
This is a really odd thing to want to do though, because you're going to end up with a map that looks like:
set_of_zones = {
instance01 = {
"asia-east1-a" => {
zone = "asia-east1-a"
}
}
}
Which doesn't seem super helpful since there is only one attribute in each map, and that attribute's value is the same as the key for that map.