datawrite.java

/* Create new object at /user-runs/$userid/$runid and at                   
* /runs/$runid simultaneously
*/
String key = mDatabase.child("runs").push().getKey();
MyRun run = new MyRun(userId, mCity, mDate, mHost, mTitle, mWeather, mWebsite, mLat,
      mLng, mLocation, mMapUrl, mTemperature, "strava", activity, photo_url);
Map<String, Object> postValues = run.toMap();

Map<String, Object> childUpdates = new HashMap<>();
childUpdates.put("/runs/" + key, postValues);
childUpdates.put("/user-runs/" + userId + "/" + key, postValues);

mDatabase.updateChildren(childUpdates).addOnSuccessListener(new OnSuccessListener<Void>() {
  @Override
  public void onSuccess(Void aVoid) {

    // sucess 
  }
});