GAE返回JSON数据

    Web page通过ajax方式调用web后台的service，返回值为json格式。我个人觉得这样的好处有以下几个方面：

1. JSON是轻量级的数据格式，相对于xml来说。
2. JSON在web page端可以方便的转为javascript对象，从而更为方面的进行数据的访问。
3. JSON格式也方便其他客户端调用，比如手机客户端，从而使web 后台service的重用性大为提高。

 

    之前的项目里用了simplejson(django.utils)，现在在网上找到一篇相关的文章：http://stackoverflow.com/questions/2114659/how-to-serialize-db-model-objects-to-json，根据这篇文章提供的json.py，可以很方便的将一个gae里的datastore查询结果转为json格式。经过测试，我发现这段代码有一个问题，即不能处理google.Query对象。为此我加了一段代码，目前使用一切正常。

elif isinstance(obj, db.Query):
  return list(obj)

 

# Copyright 2008 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
# 
#     http://www.apache.org/licenses/LICENSE-2.0
# 
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Utility classes and methods for use with simplejson and appengine.

Provides both a specialized simplejson encoder, GqlEncoder, designed to simplify
encoding directly from GQL results to JSON. A helper function, encode, is also
provided to further simplify usage.

  GqlEncoder: Adds support for GQL results and properties to simplejson.
  encode(input): Direct method to encode GQL objects as JSON.
"""

import datetime
from django.utils import simplejson
import time

from google.appengine.api import users
from google.appengine.ext import db


class GqlEncoder(simplejson.JSONEncoder):
  
  """Extends JSONEncoder to add support for GQL results and properties.
  
  Adds support to simplejson JSONEncoders for GQL results and properties by
  overriding JSONEncoder's default method.
  """
  
  # TODO Improve coverage for all of App Engine's Property types.

  def default(self, obj):
    
    """Tests the input object, obj, to encode as JSON."""

    if hasattr(obj, '__json__'):
      return getattr(obj, '__json__')()

    if isinstance(obj, db.GqlQuery):
      return list(obj)
    
    elif isinstance(obj, db.Query):
      return list(obj)

    elif isinstance(obj, db.Model):
      properties = obj.properties().items()
      output = {}
      for field, value in properties:
        output[field] = getattr(obj, field)
      return output

    elif isinstance(obj, datetime.datetime):
      output = {}
      fields = ['day', 'hour', 'microsecond', 'minute', 'month', 'second',
          'year']
      methods = ['ctime', 'isocalendar', 'isoformat', 'isoweekday',
          'timetuple']
      for field in fields:
        output[field] = getattr(obj, field)
      for method in methods:
        output[method] = getattr(obj, method)()
      output['epoch'] = time.mktime(obj.timetuple())
      return output

    elif isinstance(obj, time.struct_time):
      return list(obj)

    elif isinstance(obj, users.User):
      output = {}
      methods = ['nickname', 'email', 'auth_domain']
      for method in methods:
        output[method] = getattr(obj, method)()
      return output

    return simplejson.JSONEncoder.default(self, obj)


def encode(input):
  """Encode an input GQL object as JSON

    Args:
      input: A GQL object or DB property.

    Returns:
      A JSON string based on the input object. 
      
    Raises:
      TypeError: Typically occurs when an input object contains an unsupported
        type.
    """
  return GqlEncoder().encode(input)