firestore.rules

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    
    // match /{document=**} {
    // 	// Only allow read/write access for signed-in users
    // 	allow read, write: if request.auth != null;
    // }
    
    match /messages/{docId} {
    	allow read: if request.auth.uid != null;
      //allow write: if request.auth.uid == request.resource.data.uid
      allow create: if canCreateMessage();
    }
    
    function canCreateMessage() {
      let maxMessageLength = 5000; // characters

    	let isSignedIn = request.auth.uid != null;
      let isOwner = request.auth.uid == request.resource.data.uid;
      
      let isNotBanned = exists(
      	/databases/$(database)/documents/banned/$(request.auth.uid)
      ) == false;

      let messageLength = request.resource.data.text.size();
      let isMessageProperLength = messageLength > 0 && messageLength < maxMessageLength;
      
      return isSignedIn && isOwner && isNotBanned && isMessageProperLength;
    }
    
  }
}