[1.5.X] Fixed #19120 - Added an example of using ModelAdmin methods for read-only fields.

    By default the admin shows all fields as editable. Any fields in this

    option (which should be a ``list`` or ``tuple``) will display its data

    as-is and non-editable. This option behaves nearly identical to

    :attr:`ModelAdmin.list_display`. Usage is the same, however, when you

    specify :attr:`ModelAdmin.fields` or :attr:`ModelAdmin.fieldsets` the

    read-only fields must be present to be shown (they are ignored otherwise).

    as-is and non-editable. Note that when specifying :attr:`ModelAdmin.fields`

    or :attr:`ModelAdmin.fieldsets` the read-only fields must be present to be

    shown (they are ignored otherwise).

    If ``readonly_fields`` is used without defining explicit ordering through

    :attr:`ModelAdmin.fields` or :attr:`ModelAdmin.fieldsets` they will be

    added last after all editable fields.

    A read-only field can not only display data from a model's field, it can

    also display the output of a a model's method or a method of the

    ``ModelAdmin`` class itself. This is very similar to the way

    :attr:`ModelAdmin.list_display` behaves. This provides an easy way to use

    the admin interface to provide feedback on the status of the objects being

    edited, for example::

        class PersonAdmin(ModelAdmin):

            readonly_fields = ('address_report',)

            def address_report(self, instance):

                return ", ".join(instance.get_full_address()) or \

                   "<span class='errors'>I can't determine this address.</span>"

            # short_description functions like a model field's verbose_name

            address_report.short_description = "Address"

            # in this example, we have used HTML tags in the output

            address_report.allow_tags = True

.. attribute:: ModelAdmin.save_as

    Set ``save_as`` to enable a "save as" feature on admin change forms.