(1)  启动
           autodiscover_modules(‘admin‘, register_to=site)
           
      (2)  注册
           单例模式
           admin.site=AdminSite(): 单例对象
           
           class AdminSite(object):
                def __init__(self, name=‘admin‘):
                        self._registry = {} 
                        
                def register(self, model, admin_class=None, **options):
                    if not admin_class:
                          admin_class = ModelAdmin
                          
                    self._registry[model] = admin_class(model, self) # {Book:ModelAdmin(Book)}
                           
           
           admin.site.register(Book)  #  admin.site._registry={Book:ModelAdmin(Book)}
           admin.site.register(Publish)  #  admin.site._registry={Book:ModelAdmin(Book),Publish:ModelAdmin(Publish)}
           
           class Authoconfig(admin.ModelAdmin):
                pass
                
           admin.site.register(Author,Authoconfig)  #  admin.site._registry={Book:ModelAdmin(Book),Publish:ModelAdmin(Publish),Author:Authoconfig(Author)}