title: Update nested object in Firebase
body: <p>I'm trying to update nested object in google's Firebase Cloud Firestore.
  My database structure is:<br>
  collection 'users' <br>
  -- documents (userId)<br>
  ---- collection 'matrix<br>
  ------ document 'service' (got more documents: same type different names)<br> 
  -------- success: Story[]<br>
  -------- failure: Story[]<br>
  -------- name: string<br></p>
  
  <p>Story Model:<br> 
  <code>title: string
  text: string
  storyType: string ('success' | 'failure)
  case: string (for instance 'service')
  ...
  </code></p>
  
  <p>now I'm using Angular and I want to update a single story using. How do I do that?<br>
  I know how to get the reference to the 'service' document <br></p>
  
  <pre><code>update(story: Story, userId:string){ }
  </code></pre>
  
  <p><code>db.collection('users').doc(userId).collection('matrix).doc(story.case).update()</code><br>
  Do i need to update the whole document or can I reach the specific story by the story object (I know his type for instance 'success' and title is uniq) so I can search which has this title and update only this story? </p>