Django Admin - save_model 方法 - 如何检测字段是否已更改？

Question

我正在尝试覆盖 save_model method on a Django admin object以防止用户更改某个字段。但是，我找不到一种方法来查明该字段是否已在此方法中更改。

到目前为止，这是我的代码:

def save_model(self, request, obj, form, change):
    if change: 
        if obj.parking_location == form.cleaned_data['parking_location']:
            super(MyVehiclesAdmin, self).save_model(request, obj, form, change)
        else:
            messages.error(request, 
                "The Parking Location field cannot be changed.")

问题是 obj.parking_location 和 form.cleaned_data['parking_location'] 都有新值。 (这可能是 Django 中的错误吗？看起来 obj 应该包含预保存值)。无论如何，还有其他方法可以做到这一点吗？

(我在 Django 1.2 上)

Answer 1

首先，这不是错误，这是 Django 1.2 及更高版本中记录的行为。

来自Django 1.2 release notes :

the first time you call ModelForm.is_valid(), access ModelForm.errors or otherwise trigger form validation, your model will be cleaned in-place. This conversion used to happen when the model was saved. If you need an unmodified instance of your model, you should pass a copy to the ModelForm constructor.

如果您想阻止用户编辑特定字段，更好的方法可能是使用 ModelAdmin.readonly_fields选项。

class VehicleRegistrationAdmin(admin.ModelAdmin):
    readonly_fields = ('parking_location',)

或者，您可以将 ModelAdmin.form 替换为排除该字段的自定义表单。

class VehicleRegistrationForm(forms.ModelForm):
    class Meta:
        exclude = ('parking_location',)

class VehicleRegistrationAdmin(admin.ModelAdmin):
    form = VehicleRegistrationForm

最后，为了更直接地回答您的问题，您可以通过检查 form.changed_data 来检查 save_model 方法中的字段是否已更改。这是已更改字段名称的列表。

def save_model(self, request, obj, form, change):
    if 'parking_location' in form.changed_data:
        messages.info(request, "Parking location has changed")
    else:
        messages.info(request, "Parking location has not changed")
    super(MyVehiclesAdmin, self).save_model(request, obj, form, change)