title: How to prevent some fields of my object to be transformed by class-transformer
body: <p>I'm trying to use the library <code>class-transformer</code>(<a href="https://github.com/typestack/class-transformer" rel="noreferrer">https://github.com/typestack/class-transformer</a>) to serialize typescript classes to firebase, which doesn't accept customs types.</p>
  <p>The thing is some of the data are actually data compatible with firebase, like &quot;coordinates&quot; being in the GeoPoint format.</p>
  <p>I've this class being serialized:</p>
  <pre><code>export abstract class StopStep extends Step {
    id: string;
    name: string;
    description: string;
    pointOfInterest: PointOfInterest | null;
    address: string;
    coordinates: firebase.firestore.GeoPoint;
  }
  </code></pre>
  <p>I would like to prevent the &quot;coordinates&quot; field to be transformed. It should be present, just not untransformed.</p>
  <p>I tried to play with the <code>@Transform(({value})=&gt;...)</code>, but it appears it's only an &quot;additional&quot; transformation, it doesn't allow me to just keep the same format.</p>
  <p>I also tried <code>Exclude</code>, but then the field isn't present anymore.</p>
  <p>Is there a way to accomplish this with this library?</p>