Django model fields getter / setter

You can add a pre_save signal handler to the Model you want to save which updates the values before they get saved to the database.

It's not quite the same as a setter function since the values will remain in their incorrect format until the value is saved. If that's an acceptable compromise for your situation then signals are the easiest way to achieve this without working around Django's ORM.

Edit: In your situation standard Python properties are probably the way to go with this. There's a long standing ticket to add proper getter/setter support to Django but it's not a simple issue to resolve.

You can add the property fields to the admin using the techniques in this blog post

While I was researching the problem, I came across the solution with property decorator.

For example, if you have

class MyClass(models.Model):
    my_date = models.DateField()

you can turn it into

class MyClass(models.Model):
    _my_date = models.DateField(
        db_column="my_date",  # allows to avoid migrating to a different column
    )

    @property
    def my_date(self):
        return self._my_date

    @my_date.setter
    def my_date(self, value):
        if value > datetime.date.today():
            logger.warning("The date chosen was in the future.")
        self._my_date = value

and avoid any migrations.

Source: https://www.stavros.io/posts/how-replace-django-model-field-property/

You can also override setattr and getattr. For example, say you wanted to mark a field dirty, you might have something like this:

class MyModel:
    _name_dirty = False

    name = models.TextField()

    def __setattr__(self, attrname, val):
        super(MyModel, self).__setattr__(attrname, val)

        self._name_dirty = (attrname == 'name')


    def __getattr__(self, attrname):
        if attrname == 'name' and self._name_dirty:
            raise('You should get a clean copy or save this object.')

        return super(MyModel, self).__getattr__(attrname)

Overriding setattr is a good solution except that this can cause problems initializing the ORM object from the DB. However, there is a trick to get around this, and it's universal.

class MyModel(models.Model):
    foo = models.CharField(max_length = 20)
    bar = models.CharField(max_length = 20)

    def __setattr__(self, attrname, val):
        setter_func = 'setter_' + attrname
        if attrname in self.__dict__ and callable(getattr(self, setter_func, None)):
            super(MyModel, self).__setattr__(attrname, getattr(self, setter_func)(val))
        else:
            super(MyModel, self).__setattr__(attrname, val)

    def setter_foo(self, val):
        return val.upper()

The secret is 'attrname in self.__dict__'. When the model initializes either from new or hydrated from the __dict__!