I struggle with the way terraform handles for each loops and counts, i cant seem to figure out how to utilize the locals block to achive this. How can i grab the data disks ids from the reosurce created from a for_each loop? The vm is also from a for_each loop, i cant seem to obtain the value of managed_disk_id to be passed down to the azurerm_virtual_machine_data_disk_attachment resource block.

main.tf

    ###############################################################################################
    #                                   Linux VMs                                                 #
    ###############################################################################################
    resource "azurerm_virtual_machine" "spark-linux" {
      for_each              = var.spark_vms
      name                  = each.value.vm_name
      location              = var.location
      resource_group_name   = var.iaas_rg_name
      network_interface_ids = [azurerm_network_interface.spark-nic[each.key].id]
      vm_size               = each.value.vm_size
      availability_set_id   = azurerm_availability_set.spark_av_set.id
      tags                  = merge(var.base_tags, var.spark_vm_tags)
    
      identity {
        type = "SystemAssigned"
      }
    
      boot_diagnostics {
        enabled     = true
        storage_uri = ""
      }
    
      storage_os_disk {
        name              = "${each.value.vm_name}-osDisk"
        caching           = "ReadWrite"
        create_option     = "FromImage"
        managed_disk_type = var.prem_os_managed_disk_type
      }
    
      os_profile {
        computer_name  = each.value.vm_name
        admin_username = var.admin_username
        admin_password = var.admin_password
      }
      storage_image_reference {
        publisher = var.linux_publisher
        offer     = var.linux_offer
        sku       = var.linux_sku
        version   = var.linux_os_version
      }
    
      os_profile_linux_config {
        disable_password_authentication = false
      }
    
      delete_os_disk_on_termination    = true
      delete_data_disks_on_termination = true
    
    }
    
    resource "azurerm_managed_disk" "data" {
      for_each             = var.spark_vms
      name                 = resource "azurerm_managed_disk" "data" {
  for_each             = var.spark_vms
  name                 = format("${each.value.vm_name}-DataDisk--%02d, ${each.valuevm_data_disk_count}") 
  location             = var.location
  resource_group_name  = var.iaas_rg_name
  storage_account_type = each.value.vm_data_disk_type
  create_option        = "Empty"
  disk_size_gb         = each.value.vm_data_disk_size
  tags                 = var.spark_vm_tags
}
      location             = var.location
      resource_group_name  = var.iaas_rg_name
      storage_account_type = each.value.vm_data_disk_type
      create_option        = "Empty"
      disk_size_gb         = each.value.vm_data_disk_size
      tags                 = var.spark_vm_tags
    }
    resource "azurerm_virtual_machine_data_disk_attachment" "data" {
      for_each           = var.spark_vms
      managed_disk_id    = azurerm_managed_disk.data.*.id
      virtual_machine_id = azurerm_virtual_machine[each.value.vm_name].vm.id
      lun                = 10
      caching            = "ReadWrite"
    }

terraform.tfvars – spark_vm variables

spark_vms = {
  "vm1" = {
    vm_name            = "SPARK-VM-005"
    vm_ip_address      = "172.24.30.5"
    vm_size            = "Standard_D8s_v3"
    vm_data_disk_count = 1
    vm_data_disk_size  = 256
    vm_data_disk_type  = "Premium_LRS"
  }
  "vm2" = {
    vm_name            = "SPARK-VM-006"
    vm_ip_address      = "172.24.30.6"
    vm_size            = "Standard_D8s_v3"
    vm_data_disk_count = 1
    vm_data_disk_size  = 256
    vm_data_disk_type  = "Premium_LRS"
  }
  "vm3" = {
    vm_name            = "SPARK-VM-007"
    vm_ip_address      = "172.24.30.7"
    vm_size            = "Standard_D8s_v3"
    vm_data_disk_count = 1
    vm_data_disk_size  = 256
    vm_data_disk_type  = "Premium_LRS"
  }

}