buntsem

{
  "chat": {
     // the list of chats may not be listed (no .read permissions here)
     
     // a chat conversation
     "$key": {
         
         // if the chat hasn't been created yet, we allow read so there is a way 
         // to check this and create it; if it already exists, then authenticated 
         // user (specified by auth.id) must be in $key/users