Django admin: how to format readonly fields?

Question

I have a model, Director with two DateFields, and two subclasses (code below). I am trying to create an admin page for each Director which shows the corresponding subclass instance, and not the Director instance; this part is mostly easy (I create an inline for each subclass, give the main ModelAdmin a form with all fields excluded, and have the main ModelAdmin only request formsets from the inlines which have a corresponding instance - the code; there is an unresolved issue with this approach, which I note below, but is not the focus of this question).

The problem I have is that I want to massage the values displayed to the user, one of which is shown in a readonly field, one of which is not. The processing is that I want to change a magic value (date(1,1,1)) to the string "On incorporation".

Dates in readonly fields aren't rendered in a format very friendly to parsing, and I would like to reduce unnecessary dependence on javascript, so I would very much prefer a server-side solution.

The code below displays the forms as I want them, except that date values are not massaged at all, and when saving, there is a spurious "Please correct the error below" message, even though there are no errors, and all fields are saved correctly.

My question is: how do I intercept the values to be rendered on the page, both in readonly fields, and in forms fields, and alter them to display a string of my choosing?

The models (so far as material):

class Director(models.Model, Specializable):
    date_of_appointment = models.DateField()
    date_ceased_to_act = models.DateField(blank=True,null=True)

class DirectorsIndividual(Director):
     pass

class DirectorsCorporate(Director):
     pass

The admin code:

class DirectorAdmin(EnhancedAdmin):

    fields = ()

##    def formfield_for_dbfield(self, db_field, **kwargs):
##        return None

    def queryset(self, request):
        """ Directors for all companies which are incorporated by the current user's organisation """
        individual = Individual.for_user(request.user)
        return Director.objects.filter(company__incorporation_ticket__ordered_by__in = Organisation.all_organisations_for_which_individual_authorised_to_incorporate(individual))

    class form(forms.ModelForm):
        # have this return no html - that way only inlines are shown
        class Meta:
            fields = ()
            pass

        def is_valid(self):
            self._errors = {}
            return True

    class DirectorsIndividualInline(admin.StackedInline):
        model = DirectorsIndividual
        fk_name = 'director_ptr'
        extra = 0
        readonly_fields = ('deferred_on','company','date_of_appointment',)
        can_delete = False

        def get_readonly_fields(self, request, obj=None):
            if obj and obj.company and not obj.company.is_submitted(): return self.readonly_fields # allow editing of fields listed in else
            else:
                return itertools.chain(self.readonly_fields, ('individual', 'is_secretary'))

        def has_delete_permission(self, request, obj=None):
            return obj and ((obj.company and not obj.company.is_submitted()) or not obj.company)

        class form(forms.ModelForm):
            def __init__(self, *args, **kwargs):
                super(forms.ModelForm, self).__init__(*args, **kwargs)
                self.fields['surrogate_for'].required = False
                self.fields['representative_for'].required = False
                if self.instance:
                    obj = self.instance
                    for field in (f for f in type(obj)._meta.fields if type(f) == fields.DateField):
                        val = field.value_from_object(obj)
                        assert (type(val) in (datetime.date, type(None),))
                        # assert field.name != 'date_of_appointment'
                        if val == inc_consts.EARLIEST_DATE:
                            self.initial[field.name] = "On incorporation"

            def is_valid(self):
                self._errors = {}
                return True

    class DirectorsCorporateInline(admin.StackedInline):

        model = DirectorsCorporate
        fk_name = 'director_ptr'
        extra = 0
        can_delete = False

        class form(forms.ModelForm):
            def __init__(self, *args, **kwargs):
                super(forms.ModelForm, self).__init__(*args, **kwargs)
                if True:
                    for k in self.fields:
                        self.fields[k].required = False

            def is_valid(self):
                self._errors = {}
                return True


    inlines = (DirectorsIndividualInline,DirectorsCorporateInline)

    def get_inlines(self, request, obj=None):
        return (inline for inline in (self.inline_instances)
                if inline.model.objects.filter(**{(inline.fk_name or self.model._meta.object_name.lower()) : obj }))

    def get_formsets(self, request, obj=None):
        """ only return formset for inlines for which there exists an object """
        return (inline.get_formset(request, obj) for inline in self.get_inlines(request, obj))

I realise that there is an asymmetry between DirectorsCorporateInline and DirectorsIndividualInline; that is because I am testing on an instance with a DirectorsIndividual instance. The code above refers to model fields not shown in the models, because they are not material to the dates issue; it should be possible to render them immaterial for the spurious error issue without altering those fields (although I realise it is less helpful for that issue, I want to keep this question mostly focused on one issue). EnhancedAdmin is a ModelAdmin subclass with some minor alterations which shouldn't be of consequence. Extra code can be shown on reasoned request, but I don't want to confuse with irrelevant code.

For completeness: I am using django 1.3.1 on python 2.7.2.

Answer 1

The easiest way is to do it by defining a custom callback in the ModelAdmin. Let's say the field is called my_datetime:

from django.contrib import admin
from django.utils.formats import localize


class MyModelAdmin(admin.ModelAdmin):
    readonly_fields = ('my_datetime_localized',)

    def my_datetime_localized(self, obj):
        return localize(obj.my_datetime)
    my_datetime_localized.short_description = 'Date / time'

Note: if settings.USE_L10N is True, this will display the datetime in the local time of the viewer, which is probably what you want. If you want to keep USE_L10N as False then you can override its behaviour like so: return localize(obj.my_datetime, use_l10n=True).

Answer 2

Define a member function of your Director class that renders the readonly_field as you want.

class Director(models.Model, Specializable):
    date_of_appointment = models.DateField()
    date_ceased_to_act = models.DateField(blank=True,null=True)
    def date_of_appointment_str(self):
        if self.date_of_appointment == datetime.date(1,1,1):
            return "On incorporation"
        else:
            return "%s" % (self.date_of_appointment) # format as you wish

and then just add 'date_of_appointment_str' to your list of readonly_fields in the admin.

EDIT: I should add that this is one quick solution. A more robust solution is to subclass models.DateField into a MyCustomDateField that acts like a DateField except that when the value is date(1,1,1) it renders as "On incorporation" or when a user saves "On incorporation" it saves the value as date(1,1,1). This would ensure that you can reuse this functionality everywhere this field type shows up. However, if it only shows up in one place; this may be overkill.

You'd need something like (this is untested; you may need to additionally alter your the forms DateField and/or other things; e.g., if you use django-south you'll have to add custom introspection rules).

class MyCustomDateField(models.DateField):
    date_111_str = 'On incorporation'
    def value_to_string(self, obj):
        val = self._get_val_from_obj(obj)
        if val is None:
            data = ''
        elif val.year == val.day == val.month == 1:
            data = date_111_str
        else:
            data = datetime_safe.new_date(val).strftime("%Y-%m-%d")
        return data
    def get_prep_value(self, value):
        if value == date_111_str:
            value = datetime.date(1,1,1)
        return super(MyCustomDateField,self).get_prep_value(self, value)