Admin action for a "CSV Export" with ManyToManyField

import csv
from django.http import HttpResponse
from setuptools.compat import unicode


def export_as_csv_action(description="Export selected objects as CSV file",
						 fields=None, exclude=None, header=True):
	"""
	This function returns an export csv action
	'fields' and 'exclude' work like in django ModelForm
	'header' is whether or not to output the column names as the first row
	"""

	from itertools import chain

	def export_as_csv(modeladmin, request, queryset):
		"""
		Generic csv export admin action.
		based on http://djangosnippets.org/snippets/2369/
		"""
		opts = modeladmin.model._meta
		field_names = set([field.name for field in opts.fields])
		many_to_many_field_names = set([many_to_many_field.name for many_to_many_field in opts.many_to_many])
		if fields:
			fieldset = set(fields)
			field_names = field_names & fieldset
		elif exclude:
			excludeset = set(exclude)
			field_names = field_names - excludeset

		response = HttpResponse(content_type='text/csv')
		response['Content-Disposition'] = 'attachment; filename=%s.csv' % unicode(opts).replace('.', '_')

		writer = csv.writer(response)
		if header:
			writer.writerow(list(chain(field_names, many_to_many_field_names)))
		for obj in queryset:
			row = []
			for field in field_names:
				row.append(unicode(getattr(obj, field)))
			for field in many_to_many_field_names:
				row.append(unicode(getattr(obj, field).all()))

			writer.writerow(row)
		return response
	export_as_csv.short_description = description
	return export_as_csv