使用Python的web.py框架实现类似Django的ORM查询的教程

class Question(models.Model):
  question_text = models.CharField(max_length=200)
  pub_date = models.DateTimeField('date published')


> Question.objects.all()
> Question.objects.get(pk=1)

  # -*- coding: utf-8 -*-

  import web

  import config # 自定义的配置类，可以忽略


  def _connect_to_db():
    return web.database(dbn="sqlite", db=config.dbname)


  def init_db():
    db = _connect_to_db()
    for statement in config.sql_statements:
      db.query(statement)


  class ModelError(Exception):
    """Exception raised by all models.

    Attributes:
      msg: Error message.
    """

    def __init__(self, msg=""):
      self.msg = msg

    def __str__(self):
      return "ModelError: %s" % self.msg


  class ModelDefaultManager(object):
    """ModelManager implements query functions against a model.

    Attributes:
      cls: The class to be managed.
    """

    def __init__(self, cls):
      self.cls = cls
      self._table_name = cls.__name__.lower()

    def all(self):
      db = _connect_to_db()
      results = db.select(self._table_name)
      return [self.cls(x) for x in results]

    def get(self, query_vars, where):
      results = self.filter(query_vars, where, limit=1)
      if len(results) > 0:
        return results[0]
      else:
        return None

    def filter(self, query_vars, where, limit=None):
      db = _connect_to_db()
      try:
        results = db.select(self._table_name, vars=query_vars, where=where,
                  limit=limit)
      except (Exception) as e:
        raise ModelError(str(e))

      return [self.cls(x) for x in results]


  class ModelMetaClass(type):

    def __new__(cls, classname, bases, attrs):
      new_class = super(ModelMetaClass, cls).__new__(cls, classname,
                              bases, attrs)
      objects = ModelDefaultManager(new_class)
      setattr(new_class, "objects", objects)

      return new_class


  class Model(object):
    """Parent class of all models.
    """

    __metaclass__ = ModelMetaClass

    def __init__(self):
      pass

    def _table_name(self):
      return self.__class__.__name__.lower()

    def insert(self, **kargs):
      db = _connect_to_db()
      try:
        with db.transaction():
          db.insert(self._table_name(), **kargs)
      except (Exception) as e:
        raise ModelError(str(e))

    def delete(self, where, using=None, vars=None):
      db = _connect_to_db()
      try:
        with db.transaction():
          db.delete(self._table_name(), where, vars=vars)
      except (Exception) as e:
        raise ModelError(str(e))

    def save(self, where, vars=None, **kargs):
      db = _connect_to_db()
      try:
        with db.transaction():
          db.update(self._table_name(), where, vars, **kargs)
      except (Exception) as e:
        raise ModelError(str(e))

class Users(Model):
  ...

> user_list = Users.objects.all()