Derived Django Database Field

The trick to this is creating a custom field and overloading pre_save. Pay special attention to the self.attname member that is set to the value. The source for DateField is a good example. Make sure that if you add any new attributes to the field in it's __init__ method you also add a corresponding deconstruct method.

	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)

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