多条件排序

• • 排序在很多地方都会用到，如下图，我想排序根据等级排序
• 

• userRank = User.objects.all().order_by('-rank')[:50]

• • 一行很简单的代码，用order_by进行降序，但是这个时候万一同等级的成员咋办，比方说A和B都是Lv3的，但是B的经验值比A多50，这个时候理应说B需要排在前面，但是django会将A排在前面（因为按id的升序来排），这个时候多条件排序就很重要了
• • 一开始我认为是这样的（错误的写法）：

• userRank = User.objects.all().order_by('-rank').order_by('-Taco').order_by('-nowexp')[:50]

• • 看起来是很合理，先按等级降序，如果同等级的按Taco降序来排，如果同Taco的按当前经验降序来排，但是这样就错误了，这样写只会按最后一个排序，也就是当前经验降序来排，前面的两个都会无效
• • 然后我就去看了看这个order_by的方法

• def order_by(self, *field_names):
      """Return a new QuerySet instance with the ordering changed."""
      assert not self.query.is_sliced, \
          "Cannot reorder a query once a slice has been taken."
      obj = self._chain()
      obj.query.clear_ordering(force_empty=False)
      obj.query.add_ordering(*field_names)
      return obj

  def _chain(self, **kwargs):
      """
      Return a copy of the current QuerySet that's ready for another
      operation.
      """
      obj = self._clone()
      if obj._sticky_filter:
          obj.query.filter_is_sticky = True
          obj._sticky_filter = False
      obj.__dict__.update(kwargs)
      return obj

  def clear_ordering(self, force_empty):
      """
      Remove any ordering settings. If 'force_empty' is True, there will be
      no ordering in the resulting query (not even the model's default).
      """
      self.order_by = ()
      self.extra_order_by = ()
      if force_empty:
          self.default_ordering = False

  def add_ordering(self, *ordering):
      """
      Add items from the 'ordering' sequence to the query's "order by"
      clause. These items are either field names (not column names) --
      possibly with a direction prefix ('-' or '?') -- or OrderBy
      expressions.
  
      If 'ordering' is empty, clear all ordering from the query.
      """
      errors = []
      for item in ordering:
          if isinstance(item, str):
              if '.' in item:
                  warnings.warn(
                      'Passing column raw column aliases to order_by() is '
                      'deprecated. Wrap %r in a RawSQL expression before '
                      'passing it to order_by().' % item,
                      category=RemovedInDjango40Warning,
                      stacklevel=3,
                  )
                  continue
              if item == '?':
                  continue
              if item.startswith('-'):
                  item = item[1:]
              if item in self.annotations:
                  continue
              if self.extra and item in self.extra:
                  continue
              # names_to_path() validates the lookup. A descriptive
              # FieldError will be raise if it's not.
              self.names_to_path(item.split(LOOKUP_SEP), self.model._meta)
          elif not hasattr(item, 'resolve_expression'):
              errors.append(item)
          if getattr(item, 'contains_aggregate', False):
              raise FieldError(
                  'Using an aggregate in order_by() without also including '
                  'it in annotate() is not allowed: %s' % item
              )
      if errors:
          raise FieldError('Invalid order_by arguments: %s' % errors)
      if ordering:
          self.order_by += ordering
      else:
          self.default_ordering = False

• • order_by做了克隆，返回了一个QuerySet的副本，然后它先清空了排序，然后再设置排序
• • 看到add_ordering方法可以添加多个参数，所以就修改（正确写法）：

• userRank = User.objects.all().order_by('-rank', '-Taco', '-nowexp')[:50]

随机排序

• • 随机排序就比较简单了，还是拿上面的排行做例子
• - 方法一（我最常用的）：

• userRank = User.objects.all().order_by('?')[:50]

• - 方法二：

  userRank = list(User.objects.all())
  random.shuffle(userRank)
  userRank = userRank[:50]