如何将非模型，只写字段引入模型序列化器

我有以下ModelSerializer：

class MaintProtoSerializer（serializers.ModelSerializer）：

class Meta:                                                                 
    model = MaintenanceProtocol                                             
    fields= ('warranty_card_number', 'warranty_id', 'machine_id', 'inner_body_id',
            'outter_body_id', 'warranty', 'date', 'seller', 'inner_cleanup',
            'outter_cleanup', 'turbine_cleanup', 'filter_condition', 'pressure',
            'outer_air_temp', 'inlet_air_temp', 'other_problems', 'propuski',
            'client_notes', 'electricity')

及其型号：

class MaintenanceProtocol(models.Model):                                        
    warranty_card_number = models.CharField(max_length=100)                      
    warranty_id = models.CharField(max_length=100)                              
    machine_id = models.CharField(max_length=100)                               
    inner_body_id = models.CharField(max_length=100)                            
    outter_body_id = models.CharField(max_length=100)                                  
    warranty = models.BooleanField()                                            
    date = models.DateField()                                                   
    seller = models.CharField(max_length=100)                                   
    inner_cleanup = models.BooleanField()                                       
    outter_cleanup = models.BooleanField()                                      
    turbine_cleanup = models.BooleanField()                                            
    filter_condition = models.TextField()                                                                                                    
    pressure = models.IntegerField()                                                                                                         
    outer_air_temp = models.IntegerField()                                      
    inlet_air_temp = models.IntegerField()                                      
    other_problems = models.TextField()                                         
    electricity = models.IntegerField()                                         
    propuski = models.TextField()                                               
    client_notes = models.TextField()

它基于传递给它的以下json反序列化对象：

{
    "warranty_card_number": "profi-1234",
    "warranty_id": "fji-0938",
    "machine_id": "fuji",
    "inner_body_id": "outter-2349",
    "outter_body_id": "inner-2349",
    "warranty": "yes",
    "date": "2017-12-20",
    "seller": "maga`zin",
    "inner_cleanup": "no",
    "outter_cleanup": "yes",
    "turbine_cleanup": "yes",
    "filter_condition": "so-so",
    "pressure": "4",
    "outer_air_temp": "5",
    "inlet_air_temp": "23",
    "electricity": "45",
    "propuski": "some",
    "other_problems": "none",
    "client_notes": "n/a",
    "sigb64": "alkjhsdfpas8df,a;lsejf,p0394"
}

所以sigb64字段实际上不是模型的一部分（并且它不能持久保存到数据库中），但是我想通过序列化程序从创建的模型中访问它 . 实现这一目标的正确方法是什么？