Still on firebase database, the code shows how to update 2 nodes in the database structure. Cloned from the firebase post app.

NewPost.java

/*Please feel free to check the firbase real time databse documentation */
public class NewPost(String userId, String username, String title, String body){
  // Get[key] used to store the post at the node ["posts"]
  // [push] create the post at node["posts"] 
  String key =  mDatabase.child("posts").push.getKey();
  Post post = new Post(userId, username, title, body);
  //[Map] is a key value object in java
  // follow the link here: [https://goo.gl/e2XHLa] you should see the code where the method[toMap] is declared.
  Map<String, Object> postValues = post.toMap();
  
  Map<String, Object> childUpdates = new HashMap<>();
  childUpdates.put("/posts/" + key, postValues);
  childUpdates.put("/users-posts/" + userId + "/" + key, postValues);
  
  //The method[updateChildren] is used to update the child at a node.
  // Let me explain the dunction of push here
  
  /*
  *The [push] method generate a unique key everytime a child is added
  *The [getKey] method is used to get the key that is generated by the [push]method and this key will serve as the post id. 
  *The methd[push()] first create the post[in postValues] at the node ["posts"] for all users at /posts/$postid.
  *and simultaneously send that same post to users @/user-posts/$userid/$postid
  */
}