title: android : How to filter items in firebase by referring another database reference property
body: <p>I want to get only items that are in users favorites that has required city.
  For example, if the user has 20 favorite items and if user wants to filter favorites that exists in specific city how to filter it?</p>
  
  <p>Sorry for my English. If anything is not clear, ask me.</p>
  
  <p><img src="https://i.stack.imgur.com/GCvkz.png" alt="image"></p>