title: Get data from Firebase which contains all specified items in 'array'
body: <p>I have a structure like this:</p>
  
  <pre><code>users
   - user1
      - name: 'Bob'
      - age: 25
      - posts
         - bobPost1
            - text: 'Lorem ipsum...'
            - tags
              - web: true
              - programming: true
              - school: true
         - bobPost2
            - text: 'Lorem ipsum dolor...'
            - tags
              - web: true
              - design: true
              - school: true
         - bobPost3
            - text: 'Sample text...'
            - tags
              - web: true
              - programming: true
  </code></pre>
  
  <p>I want to retrieve all Bob's posts by tags. For example 'web' and 'school'. Result should by post1 and post2. 
  Is possible to achieve this just with firebase or should I get all his posts and then filter them? </p>