django-group-by 0.3.1

Group by arbitrary model fields

This package provides a mixin for Django QuerySets that adds a method group_by that behaves mostly like the values method, but with one difference: its iterator does not return dictionaries, but a model-like object with model instances for related values.

Installation

Install from PyPI:

pip install django-group-by

Compatibility

This package is compatible with Django 1.8, 1.9 and 1.10, and Python versions 2.7, 3.4, 3.5 and 3.6. Probably others, but those 12 combinations are the ones against which we build (by Travis CI).

Usage

Create a QuerySet subclass with the GroupByMixin to use in a model’s manager:

# models.py

from django.db import models
from django.db.models.query import QuerySet
from django_group_by import GroupByMixin

class BookQuerySet(QuerySet, GroupByMixin):
    pass

class Book(Model):
    objects = BookQuerySet.as_manager()

    title = models.CharField(max_length=100)
    author = models.ForeignKey('another_app.Author')
    publication_date = models.DateField()
...

Then use it just like values, and you’ll get a similar query set:

>>> some_rows = Book.objects.group_by('title', 'author', 'author__nationality').distinct()
>>> some_rows.count()
4

The difference is that every row is not a dictionary but an AggregatedGroup instance, with only the grouped fields:

>>> row = some_rows[0]
>>> row
<AggregatedGroup for Book>
>>> row.title
The Colour of Magic
>>> row.publication_date
*** AttributeError: 'AggregatedGroup' object has no attribute 'publication_date'

But of course the main advantage is that you also get related model instances, as far as you want:

>>> row.author
<Author: Terry Pratchett>
>>> row.author_nationality
<Nation: Great Britain>

Related Model ID Only

The previous example shows a difference in behaviour from Django’s values: we’re grouping by the foreign key author, but instead of getting only the ID we get the full instance. Why? Because it’s more useful, and I think that getting {‘author’: 5} as a result is just weird.

If you just want the ID you can specify it:

>>> some_rows = Book.objects.group_by('title', 'author_id', 'author__nationality_id').distinct()