Query-based rules with auth.uid not working

Question

I have the following structure:

events
-LEe7X118dkcH2JhJRe
    description: "Testdescr"
    eventTasks
    participants
        0zlwpSlCazNGjOyMi95h8awshrG2
            lastLogin: 1528641494535
            userKey: "0zlwpSlCazNGjOyMi95h8awshrG2"
            username: "User1"
        6LnYKxRu2SWUrxwIzId8dDpOrl02
            lastLogin: 152856590172
            userKey: "6LnYKxRu2SWUrxwIzId8dDpOrl02"
            username: "User2"

I want to query all events, to which an user participants (like a filter). My Java-Code is:

mDatabaseReference = FirebaseDatabase.getInstance().getReference("event");
mDatabaseReference.orderByChild("participants/"+userId+"/userKey").equalTo(userId).addChildEventListener [...]

This works like a charm - i only receive the correct events.

Now i want to create query-based rules on server side (https://firebase.google.com/docs/database/security/securing-data). So i created a read rule like:

".read": "query.orderByChild == 'participants/'+auth.uid+'/userKey' && query.equalTo == auth.uid"

Problem: i am not able to save that rule. Firebase gives me the error: "Invalid == expression: right operand must be an ordering or string literal when comparing against an ordering.". Why is it not possible to use "query.orderByChild" in combination with auth.uid?

What am i doing wrong?

EDIT: Here are my complete rules:

{
  "rules": {         
    ".write": true,
     "event":{
       ".read": "query.orderByChild == 'participants/'+auth.uid+'/userKey' && query.equalTo == auth.uid"      
     }
  }
}