Question

I have a deal model that contains two date field. First one is start_date and the other one is end_date. My aim is when I save a deal I want to update a readonly field which shows the substraction of these two date_field.

I try to write a widget however I can only get one of the fields. Here is my widget:

class DueToWidget(AdminDateWidget):                                                                                                                                                                        
    def render(self,name,value,attrs=None):                                                                                                                                                                
        from datetime import timedelta                                                                                                                                                                     
        output = []                                                                                                                                                                                        
        output.append(super(AdminDateWidget, self).render(name,value,attrs))                                                                                                                               
        if value:                                                                                                                                                                                          
            due_to = value + timedelta(days=1)                                                                                                                                                             
            output.append(u'<p>Diff : %s</p>' % due_to)                                                                                                                                   

        return mark_safe(u''.join(output))

I’m adding one day to the selected date, how can I get the other field’s value ? Or is there any other way to do this ?

Answer 1

If you don’t mind having to refresh to see the diff (that is, you only see it after you save the model), then an easier approach is to add a readonly field in the admin, that points to a function, like this:

class MyModelAdmin(ModelAdmin):
    readonly_fields = ('dates_difference',)

    #add your other fields, or put it in a fieldset
    fields = ('dates_difference',)

    def dates_difference(self, model_instance):
        return model_instance.end_date - model_instance.start_date

Since your goal is just to display extra information in the model’s admin this is the place to put the code, not in a field’s widget or the model’s class.

As the readonly_fields documentation specifies, its behavior is nearly identical as the list_display, that is you can point it to attributes on both the model and the model’s admin, and also to callables and methods.