title: Angular - Observable check if value exists
body: <p>I have a list of users in my database. </p>
  
  <pre><code>user1: {name: "John", pet: {name:"Riko", type: "dog"}, married: true, children: [{name: "Marry", relationship: "daughter"}] },
  user2: {name: John", married: false}
  </code></pre>
  
  <p>After fetching a user's node I wish to check if that user has certain information. In my case, I want to check if the user has the fields <code>pet</code>,<code>married</code> and <code>children</code>.</p>
  
  <p>If he does, I want to store the user's object with a new attribute inside: <code>completedQuiz: true</code>(since he has added the missing information through a quiz).</p>
  
  <p>After implementing the logic from above the result I should obtain is: </p>
  
  <pre><code>user1: {name: "John", pet: {name:"Riko", type: "dog"}, married: true, children: [{name: "Marry", relationship: "daughter"}], completedQuiz: true },
  user2: {name: John", married: false, completedQuiz: false}
  </code></pre>
  
  <p><code>user2</code> has <code>completedQuiz: false</code> since he does not have all <code>pet</code>,<code>married</code> and <code>children</code> nodes while <code>user1</code> has <code>completedQuiz: true</code> since he does have those fields.</p>
  
  <p><strong>P.S.</strong> I also want to subscribe to that list so if data is added after fetching, I will still have the most up-to-date <code>completedQuiz</code> status for all users.</p>
  
  <p>What would be a good way to implement this?</p>