title: What is the more performant option in Firestore, an array of objects or a subcollection?
body: <p>I'm thinking about the best data model of my app using Firestore. I don't know which option is more performant/scalable?
  This one:</p>
  
  <pre><code>[Collection] Estimates -&gt; 
      [Document] Estimate1 = items: [{name: 'Design', price: 200}, {name: 'Development', price: 200}]
  </code></pre>
  
  <p>Or this one:</p>
  
  <pre><code>[Collection] Estimates -&gt; 
      [Document] Estimate1 -&gt;
          [Collection] Items -&gt;
              [Document] Item 1 = {name: 'Design', price: 200}
              [Document] Item 2 = {name: 'Development', price: 200}
  </code></pre>
  
  <p>On the other hand, other than perfomance issues, I have another consideration before I choose a data model. That is that I'm going to need to copy all contents of the 'estimate' node to an 'invoice' node and I don't know if I can do this in both data models.</p>