django derived database field

models.py

from __future__ import unicode_literals

from django.db import models

class HeaderField(models.CharField):
    """
    Custom field class based on :class:`~django.db.models.CharField` that
    creates a unique header from the header, data source and site coordinates.
    """
    def pre_save(self, model_instance, add):
        """
        Create the header field the every time the model is saved.
        """
        if add:
            # if adding new record, replace tag spaces with underscores
            header = model_instance.header.replace(' ', '_')
        # XXX: if header changed later with spaces, only the first split used!
        # get the tag from header
        header = model_instance.header.split(' ', 1)
        # save latitude and longitude with 3 decimal places
        value = '%s %s %.3f %.3f' % (
            header[0], model_instance.get_data_source_display(),
            model_instance.latitude, model_instance.longitude
        )
        setattr(model_instance, self.attname, value)
        return value

# Create your models here.
class MyModel(models.Model):
    DATA_SOURCES = (
        (10, 'Meteonorm'),
        (17, 'NEDO')
    )
    data_source = models.IntegerField(choices=DATA_SOURCES)
    latitude = models.FloatField()
    longitude = models.FloatField()
    header = HeaderField(max_length=50, default='', unique=True)