How can I refer to a child wildcard variable in Firebase - firebase

Question

In an iOS messaging app I am creating, an outline of the structure for the messages node is as follows
"messages": {
"$uid":{
".read": "auth.uid == $uid",
"$messageId":{
// insert .write rule here
"toUid":{
// if its a group message then there would be multiple "$toUid" children
"$toUid":{
},
},
"fromUid":{
"$fromUid":{
}
},
"timeStamp":{
},
"group":{
"isGroupMessage":{
},
"groupId":{
}
}
}
}
},
I would like to add a write rule after $messageId to ensure that the "$toUid" or "$fromUid" is equal to the $uid .
any ideas how I can do this?
Is the following the best way to do it:
".write": "newData.child('toUid').hasChild($uid) || newData.child('fromUid').hasChild($uid)",
** I have gone with the solution below - changes structure but is the quickest / safest security rules for us to use for now.
The MVP is initially not going to have group messages (may be irrelevant but the logic could be different) - so alternatively I could add a wildCard child variable "$friendUid" and then have the following rule below that.
"$messageId":{
"$friendUid":{
".write": "$uid == auth.uid || $friendUid == auth.uid",

Answer 1

So this is how I decided to structure my Firebase database rules for my messages node. I added an extra child node with a wildcard variable of "$friendUid", in order to easily refer to it and lockdown my messages branch.
"messages": {
"$uid":{
".read": "auth.uid == $uid",
"$messageId":{
"$friendUid":{
".write": "$uid == auth.uid || $friendUid == auth.uid",
".validate": "root.child('friends/'+$uid+'/'+$friendUid).exists()",
"toUid":{
"$toUid":{
".validate": "$toUid == $friendUid || $toUid == $uid"
},
},
"fromUid":{
"$fromUid":{
".validate": "$fromUid == $friendUid || $fromUid == $uid"
},
},
"timeStamp":{
".validate":"newData.isNumber()"
},
"$other":{
".validate": false
}
}
}
}
},