Django表单外键选项初始化

问题描述

先说明一下问题的由来：
Django的模型中经常会用ForeignKey来关联其他表格数据

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)")
    # ...

使用Django的ModelForm转化为表单代码如下：

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

如果不做进一步处理，在网页中使用这个From时，关联字段会自动转化为一个select控件，里面包含了所有选项，如下图：

实际应用时，需要对关联的字段做一些选择过滤。期望的结果如下：

解决方式

在From类中设置一个初始化函数：

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

应用这个类的方式如下，注意传入参数的数据类型，

        # 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())

生成的html代码：

<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>

到这里生成的表单页面没有问题了，但是表单提交返回时如果还是用这个From来接收Request数据，则会出现数据校验失败的问题

    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)

为了解决这个问题，另外做了一个没有初始化函数的表单类来接收数据.

# 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)

感觉这里应该有更好的方法，尝试对Form的初始化函数做了一些修改，但是没有成功。

参考资料

https://qastack.cn/programming/813418/django-set-field-value-after-a-form-is-initialized

http://hk.uwenku.com/question/p-vdjpsmjn-bes.html

https://www.itranslater.com/qa/details/2325790729974580224