Django表单外键选项初始化的问题及解决方法

class MeasureTask(models.Model):
    taskname = models.CharField(max_length=LEN_FULLNAME, verbose_name="任务名称")
    road = models.ForeignKey(Road, on_delete=models.CASCADE, verbose_name="设计路段")
    # 路面层,附加一个参数 ,指定这个层的厚度,相对于底层的厚度
    # road_level = models.ForeignKey(RoadLevel, on_delete=models.CASCADE, verbose_name="路面层")
    level_thick = models.IntegerField(default=0, verbose_name="层厚(mm)")
    # ...

class MeasureTaskNewForm(forms.ModelForm):
    class Meta:
        model = MeasureTask
        fields = ('taskname', 'staff', 'start_mileage', 'end_mileage',
                  'road', 'level_thick', 'step', 'equip', 'comment')

class MeasureTaskNewForm(forms.ModelForm):
    class Meta:
        model = MeasureTask
        fields = ('taskname', 'staff', 'start_mileage', 'end_mileage',
                  'road', 'level_thick', 'step', 'equip', 'comment')

    # 对参数作初始化设置，导致返回之后的Form验证失败
    def __init__(self, road_choices=None, *args, **kwargs):
        super(MeasureTaskNewForm, self).__init__(*args, **kwargs)
        if road_choices:
            self.fields['road'].choices = road_choices

 # 对关联数据过滤
        roads = Road.objects.filter(project=p_item) 
        # 生成值，分别对应于 html 中 select->option 设置
        choices = roads.values_list('id', 'name')
        dataform = MeasureTaskNewForm(road_choices=choices)
        # dataform = MeasureTaskNewFormShadow()
        return render(request, "mdata/html/measure_task_add.html", locals())

<select name="road" id="id_road" class="form-control">
  <option value="1">北四环主线</option>
  <option value="5">匝道A</option>
</select>

if request.method == "POST":
        dataform = MeasureTaskNewForm(request.POST)
        # 这里将出现校验失败的问题
        if dataform.is_valid():
            dataform.save()
        return redirect('mdata:measure_task', pid=p_item.id)

# 影子表单模型
class MeasureTaskNewFormShadow(forms.ModelForm):
    class Meta:
        model = MeasureTask
        fields = ('taskname', 'staff', 'start_mileage', 'end_mileage',
                  'road', 'level_thick', 'step', 'equip', 'comment')

if request.method == "POST":
        dataform = MeasureTaskNewFormShadow(request.POST)
        if dataform.is_valid():
            dataform.save()
        return redirect('mdata:measure_task', pid=p_item.id)