title: Mongo { current_tracking_number: null } duplicate key error
body: <p>I'm getting this mongo duplicate key error when the following code executes. Basically, it creates a new <strong>storeAdmin</strong> document and saves it. After that, it creates a new <strong>restaurant</strong> document and assigns the previously created store admin's id to the <strong>storeAdminID</strong> field.</p>
  <p>When the restaurant collection is empty it executes without throwing an error. But when there is at least one document it gives me the above error. There is no field called <strong>current_tracking_number</strong> in any of my schemas.</p>
  <pre><code>// create a new  store admin
  const newStoreAdmin = new StoreAdmin({
    ...userData,
  });
  
  await newStoreAdmin.save();
  
  // create a new restaurant for the store admin
  const restaurant = new Restaurant({
    storeAdminID: newStoreAdmin._id,
  });
  
  await restaurant.save();
  
  // assign the newly created store admins id to the restaurant document
  
  newStoreAdmin.restaurantID = restaurant._id;
  await newStoreAdmin.save();
  </code></pre>