title: Allow anonymous user to write to firebase firestore
body: <p>I want to allow links inside my application looking like:</p>
  
  <p><code>mywebsite.com?u=nc27ri3ucfyinyh3</code></p>
  
  <p>where <code>nc27ri3ucfyinyh3</code> is a uuid, so the link can be sent to an anonymous user. The anonymous user should be able to view the page (database read), but it should also log to the database that they've viewed that link (database write).</p>
  
  <p>As we get a warning when our firestore rules look like</p>
  
  <pre><code>rules_version = '2';
  service cloud.firestore {
    match /databases/{database}/documents {
      match /{document=**} {
        allow read, write;
      }
    }
  }
  </code></pre>
  
  <p>as it is not secure.</p>
  
  <blockquote>
    <p>Your security rules are defined as public, so anyone can steal, modify or delete data in your database</p>
  </blockquote>
  
  <p>How should we handle the case of these anonymous users?</p>