ManyToManyField不显示

想要使用REST API来填充表,但是我的字段没有显示在API页面上。

型号(系列、名册):

class Series(models.Model):
    (...)
    def __str__(self):
        return self.title

class Roster(models.Model):
    (...)
    series = models.ManyToManyField(Series)
    (...)

    def __str__(self):
        return self.name

序列化程序:

class SeriesSerializer(serializers.ModelSerializer):
    class Meta:
        model = Series
        fields = ('id', 'title', 'icon')
        read_only_fields = ('slug',)

class RosterSerializer(serializers.ModelSerializer):
    series = SeriesSerializer(many=True, read_only=True)

    class Meta:
        model = Roster
        fields = ('id', 'name', 'number', 'primary_color', 'secondary_color', 'image', 'series')

意见:

class SeriesView(viewsets.ModelViewSet):
    serializer_class = SeriesSerializer
    queryset = Series.objects.all()

class RosterView(viewsets.ModelViewSet):
    serializer_class = RosterSerializer
    queryset = Roster.objects.all()

不知道我在哪里搞错了。