How to deal with “SubfieldBase has been deprecated. Use Field.from_db_value instead.”

Question

On upgrade to Django 1.9, I now get the warning

RemovedInDjango110Warning: SubfieldBase has been deprecated. Use Field.from_db_value instead.

Answer 1

While ozren is right, there is one huge difference, SubfieldBase had a really dirty side effect, it always called the to_python method on value assignment to the given model field. I have been recently bitten by this while upgrading from Django 1.9 to 1.10.

SEE:

https://docs.djangoproject.com/en/2.1/releases/1.8/#subfieldbase

Answer 2

Yes, you should just remove __metaclass__ line and add from_db_value() and to_python():

class DurationField(models.FloatField):

    def __init__(self, *args, **kwargs):
        ...

    def from_db_value(self, value, expression, connection, context):
        ...

    def to_python(self, value):
        ...

As described here: https://docs.djangoproject.com/en/1.9/ref/models/fields/#field-api-reference, to_python(value) converts the value (can be None, string or object) into the correct Python object.

from_db_value(value, expression, connection, context) converts a value as returned by the database to a Python object.

So, both methods return Python objects, but they are used by Django in different situations. to_python() is called by deserialization and during the clean() method used from forms. from_db_value() is called when the data is loaded from the database