title: Allow users to send messages to multiple users simultaneously in a messaging app
body: <p>How is a single message sent to several friends simultaneously in messaging applications? I read in a Django question that this design is a M2M relation. You define 2 models (User and SentMessage) and the backend creates a third object?</p>
  
  <p>For example, Wechat and Facebook Messenger allow you to select multiple friends and send a single message to them simultaneously. How is this done in iOS, Parse or your own Node.js backend?</p>
  
  <p>You define your classes.</p>
  
  <pre><code>user["username"] = String
  user["sex"] = String
  user["age"] = Int
  </code></pre>
  
  <p>///</p>
  
  <pre><code>let messageObj = PFObject(className: "Messages")   
  messageObj["sender"] = PFUser.current()?.username
  messageObj["message"] = messageTextView.text
  messageObj["likes"] = [String]()
  </code></pre>
  
  <p>How would you allow for the sending of messages to:<br>
  A. All users simultaneously.<br>
  B. Users with specific attributes e.g. ["age"] or ["sex"] simultaneously.</p>
  
  <p>Feel free to contribute solutions for other servers.</p>