title: How can I implement a "seen" feature using firebase for different users
body: <p>The user interacts an individual element in an array, I mark it as seen, and don't show it to the user</p>
  <p>What is the best way to implement this for multiple users? It seems fairly simple for a single user, where you can just modify the array to have a seen boolean and then just filter it, but when you have multiple users, what would be the best way to handle that?</p>
  <p>Would I create a separate &quot;elementsMarkedAsSeen&quot; file for each user? and then filter it from there? Should I store the elements in the array that the user has seen? or the elements the user has not seen? What happens if the array of elements changes?</p>
  <p>I'm just looking for a high level best practice explanation that won't cause me trouble later on. I'll do my own research from there.</p>