serializer.py

    def create(self, validated_data):

        # get the sections data
        sections = validated_data.pop('sections', [])
        user = self.context['request'].user

        # Create the chapter instance
        chapter = Chapter.objects.create(**validated_data)

        # Create each section instance
        for section in sections:
            widgets = section.pop('widgets', [])
            section.update({'chapter': chapter})
            section = Section.objects.create(**section)

            # Create each widget instance
            for widget in widgets:
                widget.update({'section': section, 'user': user})
                widget = Widget.objects.create(**widget)

        return chapter