gistfile1.py

class RelatedCompaniesField(serializers.WritableField):
    def __init__(self, *args, **kwargs):
        self.company_type = kwargs.pop('company_type')
        super(RelatedCompaniesField, self).__init__(*args, **kwargs)

    def to_native(self, obj):
        relationships = obj.relationships.filter_by_company_type(self.company_type)
        ids = list(relationships.get_orgs(self.company_type, ids_only=True))
        return ids

    def from_native(self, data):
        CompanyModel = COMPANY_MODELS_MAP[self.company_type]
        return CompanyModel.objects.filter(id__in=data)