title: Firebase security rules: restrict write only to this uid, except for a couple of fields
body: <p>In Firebase I have a <code>users</code> "node", which looks like:</p>
  
  <pre><code>users: {
    someUid: {
      username: 'someUsername'
      activeConversations: {},
      profile_picture: ''
      ... lots of other children
    },
    ...
  },
  anotherNode: {
  
  },
  ... hundreds of other nodes
  </code></pre>
  
  <p>My rules right now:</p>
  
  <pre><code>{
    "rules": {
      ".read": true,
      ".write": true,
      "users": {
        ".indexOn": [
          "username"
        ]
      },
      "friendRequests": {
        ".indexOn": [
          "timeSent"
        ]
      }
    }
  }
  </code></pre>
  
  <p>What I want to do is restrict child's access in the <code>users</code> "node" only to the client who owns the child. So for instance, the <code>someUid</code> child should only be writeable by the client with uid <code>someUid</code>. Other "node" like <code>anotherNode</code> can be writeable / readable by any logged-in client.
  Also, any logged-in client should be able to write on <code>profile_picture</code> and <code>activeConversations</code> in the <code>users</code> doc.</p>
  
  <p>How can I achieve that without having to put a read/write rule on every single node?</p>
  
  <p>Thank you</p>