@Test
public void populatesUsersToComments() {

    String id = "userId";
    CommentsFeed feed = new CommentsFeed();
    Comment comment = mock(Comment.class);
    when(comment.getUserId()).thenReturn(id);
    List<Comment> comments = Arrays.asList(comment);
    User user = mock(User.class);
    Map<String, User> users = new HashMap<>();
    users.put(id, user);
    feed.comments = comments;
    feed.users = users;
    feed.flatten();
    verify(comment).setUser(user);
}