gistfile1.py

from google.appengine.ext import db

class User(db.Model):
    nickname = db.StringProperty()

class Comment(db.Model):
    user = db.ReferenceProperty(User)
    text = db.StringProperty()

def get_commented_user_nicknames_1():
    return [comment.user.nickname for comment in Comment.all()]

def get_commented_user_nicknames_2():
    user_keys = [comment._user for comment in Comment.all()]
    users = db.get(user_keys)
    return [user.nickname for user in users]