ModelSerializer with custom fields (Django Rest Framework)

serializers.py

class CampaignSerializer(serializers.ModelSerializer):
    # companies is many-to-many field. Use StringRelatedField will show names of the companes instead of ids.
    companies = serializers.StringRelatedField(many=True)
	# format date
    created_at = serializers.DateTimeField(format="%Y-%m-%d")
	# custom field with count of companies (which is many-to-many field). It use method with the same name: get_<name of the field>(self, obj)
    companies_count = serializers.SerializerMethodField()

    def get_companies_count(self, obj):
        return obj.companies.count()

    class Meta:
        model = Campaign
        fields = ('id', 'name', 'description', 'status', 'companies', 'created_at', 'companies_count')