title: Isolate and manage collections CRUD done by signed in User - Firebase and Flutter
body: <p>I'm new to Firebase and I just need clarity on how Firebase DB works overall.</p>
  <p>Say I have different collections</p>
  <ul>
  <li>users</li>
  <li>customers</li>
  <li>items</li>
  <li>transactions</li>
  </ul>
  <p>For each of the above, I created a model class in my flutter app, each having their accompanying fields. My app is set up with FirebaseAuth that handles signIn, signOut, SignUp...</p>
  <p>How does Firebase know which customers and items belong to the currentUser()? Does it automatically store and match currentUser() uid to every CRUD data done by this user, or do I have to manually include the current user's uid as a field in each one of my model classes as in the example below?</p>
  <p>Also, say those collections/models have some dependency on each other, for example, a user can have many customers and customers may have many items in their shopping list, and many Sales could belong to a customer. What's the best way to represent this in dart / flutter ?</p>
  <p>For example, here's my Sales class</p>
  <pre><code>class Sales {
    String id;
    String totalAmount;
    Customer customerId;
    Items items;
    Timestamp createdAt;
  }
  </code></pre>
  <p>Thanks.</p>