Django ORM 查询管理器源码解析

# views.py
def index(request):
  book_set = Book.objects.filter(id=1)
  或者
  book_set = Book.objects.all()
  ......

def ensure_default_manager(sender, **kwargs):
  ......
  if not getattr(cls, '_default_manager', None):
    # Create the default manager, if needed.
    try:
      cls._meta.get_field('objects')
      raise ValueError("Model %s must specify a custom Manager, because it has a field named 'objects'" % cls.__name__)
    except FieldDoesNotExist:
      pass
    """
    关键的一步, 将一个 Manager 实例挂钩到 cls.objects, 将 model.add_to_class() 方法罗列如下;
    def add_to_class(cls, name, value):
      if hasattr(value, 'contribute_to_class'):
        value.contribute_to_class(cls, name)
      else:
        setattr(cls, name, value)
    关键是 Manager 有 contribute_to_class() 方法, 由此看来, model.objects 并不是一个 Manager 实例, 实际上他是一个 ManagerDescriptor 实例.
    """
    cls.add_to_class('objects', Manager())
    cls._base_manager = cls.objects
  elif not getattr(cls, '_base_manager', None):
    default_mgr = cls._default_manager.__class__
    if (default_mgr is Manager or
        getattr(default_mgr, "use_for_related_fields", False)):
      cls._base_manager = cls._default_manager
    else:
      # Default manager isn't a plain Manager class, or a suitable
      # replacement, so we walk up the base class hierarchy until we hit
      # something appropriate.
      for base_class in default_mgr.mro()[1:]:
        if (base_class is Manager or
            getattr(base_class, "use_for_related_fields", False)):
          cls.add_to_class('_base_manager', base_class())
          return

class ManagerDescriptor(object):
  """

  """
  # This class ensures managers aren't accessible via model instances.
  # For example, Poll.objects works, but poll_obj.objects raises AttributeError.

  def __init__(self, manager):
    self.manager = manager
  def __get__(self, instance, type=None):
    if instance != None:
      raise AttributeError("Manager isn't accessible via %s instances" % type.__name__)
    return self.manager

class Celsius(object):
  def __init__(self, value=0.0):
    self.value = float(value)
  def __get__(self, instance, owner):
    print instance,owner
    return self.value
  def __set__(self, instance, value):
    print instance,value
    self.value = float(value)

class Temperature(object):
  celsius = Celsius()

t = Temperature()
t.celsius
Temperature.celsius