for_each over complex map

test.tf

locals {
    sqs_queues = {
        usertrack = {
            topics = ["foo", "bar", "baz"]
        }

        another = {
            topics = ["asdf", "1234"]
        }
    }

    sns_topics = merge([
        for k,v in local.sqs_queues : {
            for topic in v.topics : "${k}_${topic}" => {queue = k, topic = topic}
        }
    ]...)
}

output test {
    value = local.sns_topics
}

# resource aws_sqs_queue queue {
#     for_each = var.sqs_queues
#     ...
# }

# resource aws_sns_topic topic {
#     for_each = var.sns_topics
#     ...
# }
        
 # Outputs:

# test = {
#   "another_1234" = {
#     "queue" = "another"
#     "topic" = "1234"
#   }
#   "another_asdf" = {
#     "queue" = "another"
#     "topic" = "asdf"
#   }
#   "usertrack_bar" = {
#     "queue" = "usertrack"
#     "topic" = "bar"
#   }
#   "usertrack_baz" = {
#     "queue" = "usertrack"
#     "topic" = "baz"
#   }
#   "usertrack_foo" = {
#     "queue" = "usertrack"
#     "topic" = "foo"
#   }
# }