          self.null = kwargs.pop('null', False)
          self.many = kwargs.pop('many', self.many)
          super(RelatedField, self).__init__(*args, **kwargs)
          self.read_only = kwargs.pop('read_only', self.default_read_only)
  
      def initialize(self, parent, field_name):
          super(RelatedField, self).initialize(parent, field_name)
          if self.queryset is None and not self.read_only:
 
</s> remove class ManyRelatedMixin(object):
    """
    Mixin to convert a related field to a many related field.
    """
    widget = widgets.SelectMultiple

    def field_to_native(self, obj, field_name):
        value = getattr(obj, self.source or field_name)
        return [self.to_native(item) for item in value.all()]

    def field_from_native(self, data, files, field_name, into):
        if self.read_only:
            return

        try:
            # Form data
            value = data.getlist(self.source or field_name)
        except:
            # Non-form data
            value = data.get(self.source or field_name)
        else:
            if value == ['']:
                value = []

        into[field_name] = [self.from_native(item) for item in value]


class ManyRelatedField(ManyRelatedMixin, RelatedField):
    """
    Base class for related model managers.

    If not overridden, this represents a to-many relationship, using the unicode
    representations of the target, and is read-only.
    """
    pass


 </s> add 
        if self.many:
            self.widget = self.many_widget
          self.null = kwargs.pop('null', False)
          self.many = kwargs.pop('many', self.many)
          super(RelatedField, self).__init__(*args, **kwargs)
          self.read_only = kwargs.pop('read_only', self.default_read_only)
        if self.many:
            self.widget = self.many_widget
  
      def initialize(self, parent, field_name):
          super(RelatedField, self).initialize(parent, field_name)
          if self.queryset is None and not self.read_only:
keep keep keep add keep keep keep keep
 Starting migration from ManyField to Field(many=True) 
rest_framework/relations.py