Overriding virtual fields¶

Before Django 4, it was not allowed to override a virtual field by a database field. The opposite direction was allowed already before Django 4, but in the use case presented below, Lino raised a ChangedAPI exception to avoid this pitfall.

For example, you use a library with a model mixin MyMixin, which defines a virtual field named foo. You write a model MyModel, based on MyMixin. And in your model you define yourself a field named foo. Maybe you simply didn’t know that MyMixin has already a field of that name. But your foo is a database field.

The lino_book.projects.ovfields demo application shows this by defining the following database model:

from lino.api import dd, _
from lino.mixins.ref import Referrable

class MyMixin(dd.Model):

    class Meta:
        abstract = True

    @dd.displayfield(_("Foo"))
    def foo(self, ar):
        return str(self) + "!"

class MyModel(MyMixin):

    foo = dd.CharField(_("Foo"), max_length=200, blank=True)

class MyModels(dd.Table):
    model = MyModel

    required_roles = set()

>>> from lino import startup
>>> startup('lino_book.projects.ovfields.settings')
>>> from lino.api.doctest import *

>>> # clean up previous test runs
>>> from django.core.management import call_command
>>> call_command('initdb', interactive=False, verbosity=0)

>>> obj = ovfields.MyModel(foo='bar')
>>> obj.full_clean()
>>> obj.save()
>>> print(obj)
MyModel object (1)

>>> print(obj.foo)
bar

>>> rt.show(ovfields.MyModels)
==== =====
 ID   Foo
---- -----
 1    bar
==== =====

>>> obj.foo = "new value"
>>> print(obj.foo)
new value

>>> obj.full_clean()
>>> obj.save()

>>> rt.show(ovfields.MyModels)
==== ===========
 ID   Foo
---- -----------
 1    new value
==== ===========

>>> ovfields.MyModels.get_data_elem('foo')
<lino.core.fields.CharField: foo>

>>> for ql in settings.SITE.get_quicklink_items(None):
...     print(ql)
my models

Above examples show that everything is okay, the code works as expected. Since Django 4. The issue probably was because Django before version 4 did some magic during startup, which it no longer does since version 4. When you tried to get the value of the database field, Python would call the virtual field method and give you this value.