Django Admin - save_model method - How to detect if a field has changed?

Question

I'm trying to override the save_model method on a Django admin object to prevent a user from changing a certain field. However I can't find a way to find out if the field has changed within this method.

Here's my code so far:

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.")

The problem is both obj.parking_location, and form.cleaned_data['parking_location'] have the new value. (Could this be a bug in Django? It really seems like the obj should contain the pre-save values). In any case, is there another way to accomplish this?

(I'm on Django 1.2)

Answer 1

Firstly, this isn't a bug, it's the documented behaviour in Django 1.2 onwards.

From the 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.

If you want to prevent the user from editing a paticular field, a better approach might be to use the ModelAdmin.readonly_fields option.

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

Or, you could replace the ModelAdmin.form with a custom form that excludes that field.

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

class VehicleRegistrationAdmin(admin.ModelAdmin):
    form = VehicleRegistrationForm

Finally, to answer your question more directly, you can check whether a field has changed in the save_model method by inspecting form.changed_data. This is a list of the names of the fields which have changed.

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)