在GAE中从NDB数据存储中获取第一条记录

Question

我目前正在为Google Python App Engine编写一个应用程序。我想获取数据存储区中的第一条记录，但由于某种原因，这不起作用。这是我会做它在MySQL：

SELECT * FROM MyClass LIMIT 1; 

但是因为我是新来的NDB，我不能完全似乎环绕如何工作的我的头。我尝试阅读the documentation的例子，但他们不太合适。

这是我目前拥有的代码：

from google.appengine.ext.ndb import Model as NdbModel 
from google.appengine.ext.ndb import StringProperty 


class Model(NdbModel): 

    @classmethod 
    def get_first(cls): 
     """ 
     Get the first record in the model datastore 
     :return: Model 
     """ 
     try: 
      return cls.query().fetch(1)[0] 
     except IndexError: 
      raise ModelException("No records found in [%s] datastore" % cls.__name__) 


class MyClass(Model): 
    foo = StringProperty() 
    bar = StringProperty() 

a = MyClass.get_first() # TypeError: 'NoneType' object does not support item assignment 

---

这是我的完整的堆栈跟踪：

TypeError: 'NoneType' object does not support item assignment 
    at _store_value (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/model.py:1127) 
    at _deserialize (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/model.py:1496) 
    at _from_pb (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/model.py:3209) 
    at pb_to_entity (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/model.py:691) 
    at pb_to_query_result (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/datastore/datastore_rpc.py:201) 
    at _process_results (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/datastore/datastore_query.py:2980) 
    at __query_result_hook (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/datastore/datastore_query.py:2947) 
    at get_result (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/api/apiproxy_stub_map.py:613) 
    at _on_rpc_completion (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py:513) 
    at _run_to_list (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/query.py:995) 
    at _help_tasklet_along (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py:427) 
    at get_result (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/tasklets.py:383) 
    at fetch (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/query.py:1218) 
    at positional_wrapper (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/ext/ndb/utils.py:160) 
    at get_first (/base/data/home/apps/s~project-id/v1.395271489319521947/project/core/models/utils.py:55) 
    at me (/base/data/home/apps/s~project-id/v1.395271489319521947/project/core/routes/api/v1/profile.py:22) 
    at dispatch_request (/base/data/home/apps/s~project-id/v1.395271489319521947/lib/flask/app.py:1625) 
    at full_dispatch_request (/base/data/home/apps/s~project-id/v1.395271489319521947/lib/flask/app.py:1639) 
    at handle_user_exception (/base/data/home/apps/s~project-id/v1.395271489319521947/lib/flask/app.py:1544) 
    at full_dispatch_request (/base/data/home/apps/s~project-id/v1.395271489319521947/lib/flask/app.py:1641) 
    at wsgi_app (/base/data/home/apps/s~project-id/v1.395271489319521947/lib/flask/app.py:1988) 
    at handle_exception (/base/data/home/apps/s~project-id/v1.395271489319521947/lib/flask/app.py:1567) 
    at wsgi_app (/base/data/home/apps/s~project-id/v1.395271489319521947/lib/flask/app.py:1991) 
    at __call__ (/base/data/home/apps/s~project-id/v1.395271489319521947/lib/flask/app.py:2000) 
    at Handle (/base/data/home/runtimes/python27/python27_lib/versions/1/google/appengine/runtime/wsgi.py:267) 

Answer 1

好像你正试图立即检索数据是经过写入数据存储区，除非您在App Engine的Storing Data in Cloud Datastore文档中所述的每个实体上设置相同的parent key，否则该数据存储区将无法工作。

要创建parent key，你可以写：

from google.appengine.ext.ndb import Key 
parent_key = Key('my_data', 'version_1') 

，当你创建你的实体，你应该通过parent key到constructor：

my_entity = MyKindClass(parent=parent_key, foo=foo_input, bar=bar_input) 

然后，通过传递查询数据库您的kind类别的parent_key至query方法：

first_result = MyKindClass.query(
      ancestor=parent_key).order(-MyKindClass.created).get() 

我认为你有兴趣获得最新的数据，所以我使用order方法对结果进行了排序，-运算符下令对数据进行排序。另外，如果没有数据，我使用get方法而不是fetch(1)返回第一个查询结果或None。

完整的代码将是：

import webapp2 
from google.appengine.ext.ndb import (
    Model, StringProperty, DateTimeProperty, Key) 


class MyKindClass(Model): 
    foo = StringProperty() 
    bar = StringProperty() 
    created = DateTimeProperty(auto_now_add=True) 


parent_key = Key('my_data', 'version_1') 


class MainHandler(webapp2.RequestHandler): 

    def get(self): 
     foo_input = self.request.get('foo') 
     bar_input = self.request.get('bar') 
     if foo_input and bar_input: 
      my_entity = MyKindClass(parent=parent_key, 
            foo=foo_input, bar=bar_input) 
      my_entity.put() 

     first_result = MyKindClass.query(
      ancestor=parent_key).order(-MyKindClass.created).get() 
     if first_result is not None: 
      self.response.write('foo: %s, and bar: %s' % 
           (first_result.foo, first_result.bar)) 
     else: 
      self.response.write('No data yet!') 

app = webapp2.WSGIApplication([ 
    ('/', MainHandler) 
], debug=True) 

欲了解更多信息，请参阅App Engine的The Python NDB Client Library Overview文件。