Django中的對象查詢

Django框架自帶了ORM，實現了一些比較強大而且方便的查詢功能，這些功能和表無關。比如下面這個例子：

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)

從例子可以看出，objects.all和objects.get這些功能都不是在class Question中定義的，可能在其父類models.Model中定義，也可能不是。那么我們在web.py中如何實現這樣的功能呢？（如果你選擇使用SQLAlchemy就不需要自己實現了）。
實現
思路

我們注意到Question.objects.all()這樣的調用是直接訪問了類屬性objects，并調用了objects屬性的方法all()。這里objects可能是一個實例，也可能是一個類。我個人認為（我沒看過Django的實現）這應該是一個實例，因為實例化的過程可以傳遞一些表的信息，使得類似all()這樣的函數可以工作。經過分析之后，我們可以列出我們需要解決的問題：

代碼

都說不給代碼就是耍流氓，我還是給吧。說明下：使用的數據庫操作都是web.py的db庫中的接口。

  # -*- coding: utf-8 -*-  import web  import config # 自定義的配置類，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))