Firebase Rules wildcard Key

I've searched the forums for an answer to this but can't find one.
I'm writing a Key:value pair to firebase database and want to set rule.

It's a chat application, and I'm adding a field to my user for each chat he's a member of, eg user/$uid/chat/$chat_id

The chatId i'm creating using

FirebaseDatabase.getInstance().getReference().child("chat").push().getKey();

I'm trying to add some kind of validation so that the data has to contains $chat_id : true.
But I'm strugging to get the wildcard to work.

The test data I'm writing -
"-M5STgMVWyafV-LhPIT2": "true", but I need the Key to be a wildcard as it's random each time.

The data structure looks like this.

chat
	-M5STgMVWyafV-LhPIT2   // <<< $chat_id
		info
			admin: "zQvmGG2vPfTrdjZBfWP2ZotajYE2"
			groupName: "Some Group"
			id: "-MJD6cAtpxY_jmFyk-sb"
			no_of_users: 2
			timer: 60000
			users
				y1EKihfmIJTDCD9OJt0N89WDC642: true
				zQvmGG2vPfTrdjZBfWP2ZotajYE2: true
		messages
			-MJ0G0gdecak8ACR7fb2
				creator: "wjl8VNmcaPNHJAUQnCDRHWkJ33p2"
				date: "Oct 06, 2020"
				text: "Some Text"
				time: "23:13"
				userName: "Bob"

user
	zQvmGG2vPfTrdjZBfWP2ZotajYE2
		chat
			-M5STgMVWyafV-LhPIT2: true  // <<< $chat_id
			-MGNk6fl8jneDhw7Jv02: true
			-MGNkFGD4OYb7ZSzX8P2: true
			-MGNl47kbPuNZFc3RLD2: true
			-MGNyFIuhjrwyQT4VME2: true
			-MGNydtnFHW60i5TbzW2: true
			-MJ-ETJ-LcOxvYkZ8b72: true

		info
			email: "someemailaddress@me.com"
			name: "Bob"
			notificationKey: "be5df5fa-3a20-4704-986f-8a35d"
			profilePic: "https://"
			status: "Some status"
	zQvmGG2vPfTrdjZBfWP2ZotajYE3
	zQvmGG2vPfTrdjZBfWP2ZotajYE4

And the rules I've tried, which work if I specify the $chat_id as a string.

{
  "rules": {
    "user": { 
      ".read": "auth != null",
        ".indexOn": "info/email",
      "$user_id": {
          
           		"chat": {        					
                ".write": "auth != null",
                ".validate": "newData.child($chat_id).val() == 'true'", // <<< this doesn't work Unknown variable '$chat_id'.
                //".validate": "newData.child("-M5STgMVWyafV-LhPIT2").val() == 'true'",  <<< this works, but not $chat_id, I need this Child to be a wildcard

              },
               "info": { 
                //".indexOn": "email", 
                ".write": "auth != null && $user_id === auth.uid",
              },
         	}
   		},
    "chat": {
     //".write": "auth != null && !data.child('chat').child('$chat_id').exists()",
    "$chat_id": {
          "info": {
            ".read": "data.child('users').child(auth.uid).exists()",
            ".write": "auth != null && !data.exists()&&newData.child('admin').val()==auth.uid || data.child('users').child(auth.uid).exists()",
          },
          "messages": {
              "$message_id": {
                ".write": "root.child('chat').child($chat_id).child('info').child('users').child(auth.uid).exists()",
                ".read": "root.child('chat').child($chat_id).child('info').child('users').child(auth.uid).exists()",    
             }
        }
      }
    },
  }
}

Any help would be apreciated.
Thanks

This:

newData.hasChild('$chat_id')

Means that you're checking if the new data has a child node that is literally called $chat_id. Aside from the fact that this can't be true (as $ is not allowed in keys), it is also likely not what you want.

If you instead want to check if the new data has a child with the same value as $chat_id in your database, use $chat_id without quotes. So:

newData.hasChild($chat_id)

I figured out the correct syntax for this. I needed to add the variable after the write function, and then put the validate after the variable.

Here it is in case it helps anyone later.

      "$user_id": {
            "chat": {        					
                  ".write": "auth != null",
                "$chat_id":    {
                  ".validate": "$chat_id.length === 20 && newData.val() === 'member'",
                },
                  },