Merging complex types - terraform

Question

I'm struggling with merging maps that have duplicate keys, since the built-in merge function will only keep the latest argument that has matching keys or attributes.
My maps are of the following shape:
{mykey = ["item1","item2","item3"]}
{mykey = ["item4","item5","item6"]}
The merge function simply returns {mykey = ["item4","item5","item6"]} (or whatever is the last argument). I'd like to compose a map like
{mykey = ["item1","item2","item3","item4","item5","item6"]}
I don't believe there's a function I can use to achieve this. However, I suspect that a for loop is the right approach, yet my knowledge is failing me.
Hi #MattSchuchard, thanks for your help. The parent map structure (if I am following you correctly), is a local variable. I am using keys and values from this local in order to retrieve object_ids from the azuread_users and azuread_groups data sources. My locals looks like this:
locals {
roles = {
role1 = {
users = ["user1#domain.com","user2#domain.com"]
groups = ["myGroup1","myGroup2"]
}
}
I am referencing this local in the data sources as follows:
data azuread_users.lookup {
for_each = local.roles
user_principal_names = each.values.users
}
data azuread_groups.lookup {
for_each = local.roles
display_names = each.values.groups
}
These data sources export object_ids as an attribute (which I will then use when adding members to groups in a resource block). The exported attributes are in the shape detailed in my original post.
I would like to compose a combined list of all object_ids exported from both data.azuread_users.lookup and data.azuread_groups.lookup which I can then provide as an argument when creating a group:
resource "azuread_group" "my_group" {
members = $myCombinedListOfObjectIDs
}

Answer 1

Depending on whether you want a list/map you can use this:
locals {
input = { mykey = ["item1", "item2", "item3"] }
input2 = { mykey = ["item4", "item5", "item6"] }
merged = concat(local.input.mykey, local.input2.mykey)
merged_mykey = {
mykeys = concat(local.input.mykey, local.input2.mykey)
}
}
The output is
> local.merged_mykey
{
"mykeys" = [
"item1",
"item2",
"item3",
"item4",
"item5",
"item6",
]
}
> local.merged
[
"item1",
"item2",
"item3",
"item4",
"item5",
"item6",
]
If you're worried about duplicates you can wrap it in a distinct call. https://www.terraform.io/language/functions/distinct