Maison > Article > développement back-end > 使用Python的web.py框架实现类似Django的ORM查询的教程
Django中的对象查询
Django框架自带了ORM,实现了一些比较强大而且方便的查询功能,这些功能和表无关。比如下面这个例子:
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)
从例子可以看出,objects.all和objects.get这些功能都不是在class Question中定义的,可能在其父类models.Model中定义,也可能不是。那么我们在web.py中如何实现这样的功能呢?(如果你选择使用SQLAlchemy就不需要自己实现了)。
实现
思路
我们注意到Question.objects.all()这样的调用是直接访问了类属性objects,并调用了objects属性的方法all()。这里objects可能是一个实例,也可能是一个类。我个人认为(我没看过Django的实现)这应该是一个实例,因为实例化的过程可以传递一些表的信息,使得类似all()这样的函数可以工作。经过分析之后,我们可以列出我们需要解决的问题:
代码
都说不给代码就是耍流氓,我还是给吧。说明下:使用的数据库操作都是web.py的db库中的接口。
# -*- 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))
使用
首先定义表对应的类:
class Users(Model): ...
使用就和Django的方式一样:
>>> user_list = Users.objects.all()