Django Order By Date, but have “None” at end?

Question

I have a model of work orders, with a field for when the work order is required by. To get a list of work orders, with those that are required early, I do this:

Answer 1

Requirements: Python 3.4, Django 10.2, PostgreSQL 9.5.4

Variant 1

Solution:

class IsNull(models.Func):

    template = "%(expressions)s IS NULL"

Usage (None always latest):

In [1]: a = User.polls_manager.users_as_voters()

In [4]: from django.db import models

In [5]: class IsNull(models.Func):
   ...:     template = "%(expressions)s IS NULL"
   ...:     

In [7]: a = a.annotate(date_latest_voting_isnull=IsNull('date_latest_voting'))

In [9]: for i in a.order_by('date_latest_voting_isnull', 'date_latest_voting'):
   ...:     print(i.date_latest_voting)
   ...:     
2016-07-30 01:48:11.872911+00:00
2016-08-31 13:13:47.240085+00:00
2016-09-16 00:04:23.042142+00:00
2016-09-18 19:45:54.958573+00:00
2016-09-26 07:27:34.301295+00:00
2016-10-03 14:01:08.377417+00:00
2016-10-21 16:07:42.881526+00:00
2016-10-23 11:10:02.342791+00:00
2016-10-31 04:09:03.726765+00:00
None

In [10]: for i in a.order_by('date_latest_voting_isnull', '-date_latest_voting'):
    ...:     print(i.date_latest_voting)
    ...:     
2016-10-31 04:09:03.726765+00:00
2016-10-23 11:10:02.342791+00:00
2016-10-21 16:07:42.881526+00:00
2016-10-03 14:01:08.377417+00:00
2016-09-26 07:27:34.301295+00:00
2016-09-18 19:45:54.958573+00:00
2016-09-16 00:04:23.042142+00:00
2016-08-31 13:13:47.240085+00:00
2016-07-30 01:48:11.872911+00:00
None

Notes

1. Based on https://www.isotoma.com/blog/2015/11/23/sorting-querysets-with-nulls-in-django/
2. Drawback: unnecessary buffer field, overhead for ordering

Variant 2

Solution:

from django.db import models
from django.db import connections
from django.db.models.sql.compiler import SQLCompiler


class NullsLastCompiler(SQLCompiler):

    # source code https://github.com/django/django/blob/master/django/db/models/sql/compiler.py

    def get_order_by(self):

        result = super(NullsLastCompiler, self).get_order_by()

        # if result exists and backend is PostgreSQl
        if result and self.connection.vendor == 'postgresql':

            # modified raw SQL code to ending on NULLS LAST after ORDER BY
            # more info https://www.postgresql.org/docs/9.5/static/queries-order.html
            result = [
                (expression, (sql + ' NULLS LAST', params, is_ref))
                for expression, (sql, params, is_ref) in result
            ]

        return result


class NullsLastQuery(models.sql.Query):

    # source code https://github.com/django/django/blob/master/django/db/models/sql/query.py
    def get_compiler(self, using=None, connection=None):
        if using is None and connection is None:
            raise ValueError("Need either using or connection")
        if using:
            connection = connections[using]

        # return own compiler
        return NullsLastCompiler(self, connection, using)


class NullsLastQuerySet(models.QuerySet):

    # source code https://github.com/django/django/blob/master/django/db/models/query.py
    def __init__(self, model=None, query=None, using=None, hints=None):

        super(NullsLastQuerySet, self).__init__(model, query, using, hints)

        # replace on own Query
        self.query = query or NullsLastQuery(model)

Usage:

# instead of models.QuerySet use NullsLastQuerySet
class UserQuestionQuerySet(NullsLastQuerySet):

    def users_with_date_latest_question(self):

        return self.annotate(date_latest_question=models.Max('questions__created'))


#connect to a model as a manager
class User(AbstractBaseUser, PermissionsMixin):
    .....

    questions_manager = UserQuestionQuerySet().as_manager()

Results (None always latest):

In [2]: qs = User.questions_manager.users_with_date_latest_question()

In [3]: for i in qs:
   ...:     print(i.date_latest_question)
   ...:     
None
None
None
2016-10-28 20:48:49.005593+00:00
2016-10-04 19:01:38.820993+00:00
2016-09-26 00:35:07.839646+00:00
None
2016-07-27 04:33:58.508083+00:00
2016-09-14 10:40:44.660677+00:00
None

In [4]: for i in qs.order_by('date_latest_question'):
   ...:     print(i.date_latest_question)
   ...:     
2016-07-27 04:33:58.508083+00:00
2016-09-14 10:40:44.660677+00:00
2016-09-26 00:35:07.839646+00:00
2016-10-04 19:01:38.820993+00:00
2016-10-28 20:48:49.005593+00:00
None
None
None
None
None

In [5]: for i in qs.order_by('-date_latest_question'):
   ...:     print(i.date_latest_question)
   ...:     
2016-10-28 20:48:49.005593+00:00
2016-10-04 19:01:38.820993+00:00
2016-09-26 00:35:07.839646+00:00
2016-09-14 10:40:44.660677+00:00
2016-07-27 04:33:58.508083+00:00
None
None
None
None
None

Notes:

1. Based on the Django: Adding "NULLS LAST" to query and the Django`s source code

2. Global on all fields of a model (it is advantage and disadvantage simultaneously)

3. No a unnecessary field

4. A drawback - tested only on the PostgreSQL