title: How can I query multiple fields on a single map data in Firestore?
body: <p>I'm building an app using React + Redux + Firebase. In Firestore users collection, I have multiple fields including language, subject as map data. So for example,</p>
  
  <pre><code>// language that user can speak
  language = {
    english: true
  }
  // subjects that the user can teach
  subject = {
    math: true
    science: true
  }
  </code></pre>
  
  <p>In the main app, user can search other users by filtering subjects and languages. For example, user may look for the other users who can teach 'math' and 'science' using an 'english' language.</p>
  
  <p>And I'm struggling with filtering multiple fields in map data. I could query for on field by doing <code>ref.where('subject.${val}', '==' , true)</code>. But is there any way to query on multiple fields to get a document which contains a subject hash at any given number of subjects? </p>
  
  <p>I tried to put data in array but currently firebase Array membership only support filtering one item in the array. So I guess it's a good start with storing in hash map. Any suggestion?</p>