title: Firestore Document Rules on Field
body: <p>I am looking to apply firestore rules to restrict the change (write, update) of a specific field in a document.</p>
  
  <p>From what I understand in the docs you cannot apply rules on <code>read:</code> to document fields as documents need to be read in their entirety, however, it's not stated about writes, updates?</p>
  
  <p>My structure is like the below example;</p>
  
  <pre><code>match /ads/{adDocument} {
  
      //adDocument has a field "price" this needs to only be read not changed/updated.
  
  };
  </code></pre>
  
  <p>How would I go about implementing this?</p>