Firebase Python“Reference”对象没有“limit\u to\u last”属性

import firebase_admin
enter code here`from firebase_admin import credentials
from firebase_admin import db

import firebase_admin
from firebase_admin import credentials
from firebase_admin import db
cred = credentials.Certificate('path/to/serviceAccountKey.json')
firebase_admin.initialize_app(cred, {
    'databaseURL': 'https://databaseName.firebaseio.com'
})
ref = db.reference('dinosaurs')
snapshot = ref.order_by_child('height').limit_to_last(2).get()
for key in snapshot:
    print (key)

我已经按照 this page 直截了当,但我得到了这个错误:

'Reference' object has no attribute 'limit_to_last'

我已确保正确安装Python Admin SDK。以前有人遇到过这个问题吗?