Reference referenced resources in loop in Terraform

I have existing ServiceBus topics and i want to reference their subscriptions in terraform. The question is how to do it when i have multiple topics and i wnat to build reference to subscription in each topic?

data "azurerm_servicebus_topic" "subscribe_topics" {
  for_each     = local.subscribe_topics

  name         = each.value
  namespace_id = data.azurerm_servicebus_namespace.sb.id
}

data "azurerm_servicebus_subscription" "subscriptions" {
  topic_id = // how to iterate over topic ids from data block above?
  name     = local.sb_subscription_name
}

locals {
  subscribe_topics = [
    "topic1",
    "topic2",
    "topic3"
  ]

  sb_subscription_name = "xyz"
}

The first thing I notice is your subscribe_topics is not suitable for a loop, if you try it like that it will give you an error:

Error: Invalid for_each argument

The given "for_each" argument value is unsuitable: the "for_each" argument must be a map, or set of strings, and you have provided a value of type tuple.

the fix is easy for_each = toset(local.subscribe_topics)

I don't have those resources to confirm, but all you need is to loop over the topic:

data "azurerm_servicebus_topic" "subscribe_topics" {
  for_each     = toset(local.subscribe_topics)

  name         = each.value
  namespace_id = data.azurerm_servicebus_namespace.sb.id
}

data "azurerm_servicebus_subscription" "subscriptions" {
  for_each = data.azurerm_servicebus_topic.subscribe_topics

  topic_id = each.value.id
  name     = local.sb_subscription_name
}

locals {
  subscribe_topics = [
    "topic1",
    "topic2",
    "topic3"
  ]

  sb_subscription_name = "xyz"
}
 

and another way

data "azurerm_servicebus_topic" "subscribe_topics" {
  for_each     = toset(local.subscribe_topics)

  name         = each.value
  namespace_id = data.azurerm_servicebus_namespace.sb.id
}

data "azurerm_servicebus_subscription" "subscriptions" {
  for_each = toset(local.subscribe_topics)

  topic_id = data.azurerm_servicebus_topic.subscribe_topics[each.key].id
  name     = local.sb_subscription_name
}

locals {
  subscribe_topics = [
    "topic1",
    "topic2",
    "topic3"
  ]

  sb_subscription_name = "xyz"
}