Entity_id for auto-discovered chromecast devices?

My apologies if this is covered elsewhere. I’m new here, still swimming a bit.

I’d like to set up some automation to play audio on my Chromecast devices. Some examples of the cast names I have are “Bedroom Speaker” and “Home Group” (the latter of which is a group of 4 devices)

Here’s an example automation I’d like to do:

- alias: Morning Radio
  initial_state: True
  hide_entity: False
    platform: time
    at: '7:30:00'    
    service: media_player.play_media
      entity_id: media_player.XXXXXX
      media_content_id: http://kpbs.streamguys1.com/kpbs-aac
      media_content_type: 'audio/mp3'

My question is: how do I specify the entity_id? I’ve looked into the code and the docs on the cast component, and there doesn’t seem to be any way to specify a symbolic name. Can I use the string “Home Group?” This is an even more weird case, since it’s not a single device.

OK, just experimented and figured it, sort of.

For reference, it seems there an auto-magical symbol creation, where:

“Home Group” ==> media_player.home_group
“Bedroom Speaker” ==> media_player.bedroom_speaker

Very nice, but non-obvious!