title: Allow writing to specific fields only to paid users?
body: <p>I have a document in which all users can write into, but I would like to limit writing to <strong>specific</strong> fields or specific data in this document <strong>only</strong> to paid users.</p>
  
  <p>I know how to create a rule to allow writing to a certain document based on a certain field - but how to make this rule valid only when you write to a certain field? ( not any write)</p>
  
  <pre><code> allow write,update: if request.resource.data.brand.userID ==  request.auth.uid;
  </code></pre>
  
  <p>How would I allow writing a certain field <code>logo</code> ( <code>{"logo":"me.png"}</code> ) to a document, only if the written document contains field <code>"paid":true</code> . (but keep allowing other fields to be written free)</p>
  
  <p>Or - is there <em>best practice</em> way to do this ?</p>