title: Comparing resource and request resource in firestore
body: <p>I have a simple firestore rule:</p>
  
  <p><code>allow update: if request.resource.data.reservedBy == resource.data.reservedBy;</code></p>
  
  <p>my payload in the simulator is </p>
  
  <p>{"<strong>name</strong>":"/databases/(default)/documents/books/I3dbzzwGJGXnqMQBOxoP","data":{"reservedBy":"Ivanko"}}</p>
  
  <p>and the method is "update". The value of the field 'reservedBy' is completely different in the actual record.</p>
  
  <p>No matter what I do, no matter what i change the payload to, the rule will always return true and allow the update. Is that a bug or am i missing something. Interestingly, if I change "update" to "create", the rule works as expected. Anyone else encountered this? </p>
  
  <p>Thanks.</p>