How to resize VM disk with RbVmomi in Ruby

I'm working on a project where I'm cloning a VM with RbVmomi and after the clone is finished I want to ReConfig the VM.

The cloning is working great, but I have some problems when I want to change the disk size of the VM. I can change the numbers of CPUs and the size of memory, but when I try to change the disk size I get the error:

RbVmomi::Fault (InvalidDeviceSpec: Invalid configuration for device '0'.):

Code:

if json.has_key?('CPU_COUNT') && json.has_key?('RAM')
  vm_cfg = {
      :numCPUs => json['CPU_COUNT'],
      :memoryMB => json['RAM'],
      :deviceChange => [
          :operation => :edit, #also tried with :add
          :fileOperation => :create,
          :device => RbVmomi::VIM.VirtualDisk(
              :key => 0,
              :backing => RbVmomi::VIM.VirtualDiskFlatVer2BackingInfo(
              :fileName => '[datastore]',
              :diskMode => :persistent,
              :thinProvisioned => true
          ),
          :controllerKey => 1000,
          :unitNumber => 0,
          :capacityInKB => json['DISK_SIZE'] * 1024 * 1024
      )
  ]
  }

  dc.find_vm(vmID).ReconfigVM_Task(:spec => vm_cfg).wait_for_completion

Since you are trying to reconfigure an existing VirtualMachine, you could do something like this:

#Get the disk from the VM (assuming you only have one disk)
disk = vm.config.hardware.device.grep(RbVmomi::VIM::VirtualDisk).first

#Set new capacity
disk.capacityInKB = new_capacity_in_kb

#Add the disk to the devicechange, specifying operation edit
vm_cfg = {
    :deviceChange => [
        {
            :device => disk,
            :operation => :edit
        }
    ]
}

#Start the ReconfigVM_Task with the disk edit on the VM
vm.ReconfigVM_Task(:spec => vm_cfg).wait_for_completion