Django and PostgreSQL composite types

PostgreSQL has this nifty feature called composite types that you can use to create your own types from the built-in PostgreSQL types. It’s a bit like hstore, only structured, which makes it great for structured data that you might reuse multiple times in a model, like addresses.

Unfortunately to date, they were pretty much a pain to use in Django. There were some older implementations for versions of Django before 1.7, but they tended to do things like create surprise new objects in the namespace, not be migrateable, and require connection to the DB at any time (i.e. during your build).

Anyway, after reading a bunch of their implementations and then the Django source code I wrote django-postgres-composite-types.

Install with:

pip install django-postgres-composite-types

Then you can define a composite type declaratively:

from django.db import models
from postgres_composite_type import CompositeType

class Address(CompositeType):
    """An address."""

    address_1 = models.CharField(max_length=255)
    address_2 = models.CharField(max_length=255)

    suburb = models.CharField(max_length=50)
    state = models.CharField(max_length=50)

    postcode = models.CharField(max_length=10)
    country = models.CharField(max_length=50)

    class Meta:
        db_type = 'x_address'  # Required

And use it in a model:

class Person(models.Model):
    """A person."""

    address = Address.Field()

The field should provide all of the things you need, including formfield etc and you can even inherit this field to extend it in your own way:

class AddressField(Address.Field):
    def __init__(self, in_australia=True, **kwargs):
        self.in_australia = in_australia


Finally to set up the DB there is a migration operation that will create the type that you can add:

import address
from django.db import migrations

class Migration(migrations.Migration):

    operations = [
        # Registers the type
            field=address.Address.Field(blank=True, null=True),

It’s not smart enough to add it itself (can you do that?). Nor would it be smart enough to write the operations to alter a type. That would be a pretty cool trick. But it’s useful functionality all the same, especially when the alternative is creating lots of 1:1 models that are hard to work with and hard to garbage collect.

It’s still pretty early days, so the APIs are subject to change. PRs accepted of course.

Filtering derived fields with Wagtail search

Wagtail’s built in search functionality has this nifty feature where it will index callables on your model, e.g. your model has a start and end date, but you want to search on duration. Hypothetically we could add a FilterField here to index this for Elasticsearch1.

class Job(Model, index.Indexed):
    """A job you can apply for."""

    start_date = models.DateField()
    end_date = models.DateField()

    search_fields = (
        index.FilterField('duration', type='IntegerField'),

    def duration(self):
        """Duration of the assignment."""
        return 12 * (self.end_date.year - self.start_date.year) + \
            self.end_date.month - self.start_date.month

Wagtail is quite clever in that it takes a Django QuerySet and decomposes the filters.

queryset = queryset\
    .filter(duration__range=(data['duration'].lower or 0,
                             data['duration'].upper or 99999))
query =, queryset)

Of course Django will get upset about this, since that’s not a field you can filter on. So we can annotate the value in.

from django.db.models import Func, fields

class DurationInMonths(Func):  # pylint:disable=abstract-method
    SQL Function to calculate the duration of assignments in months.
    template = \
        'EXTRACT(year FROM age(%(expressions)s)) * 12 + ' \
        'EXTRACT(month FROM age(%(expressions)s))'
    output_field = fields.IntegerField()

queryset = queryset\
        F('end_date'), F('start_date')
    .filter(duration__range=(data['duration'].lower or 0,
                             data['duration'].upper or 99999))

Also Django will be upset it can’t annotate the duration, so you’ll need to add a setter to your model.

class Job(Model, index.Indexed):
    def duration(self):

    def duration(self, value):
        """Ignored to make Django annotations happy."""

And you ideally would be done except now Wagtail gets upset that it can’t determine the attribute name for the field but you can kludge around this for now:

class DurationInMonths(Func):  # pylint:disable=abstract-method
    SQL Function to calculate the duration of assignments in months.
    template = \
        'EXTRACT(year FROM age(%(expressions)s)) * 12 + ' \
        'EXTRACT(month FROM age(%(expressions)s))'
    output_field = fields.IntegerField()
    target = type('IntegerFieldKludge',
                  {'attname': 'duration'})()
  1. Note the requirement for a type, otherwise this will be indexed as a string. []

Multiple choice using Django’s postgres ArrayField

There’s a lot of times you want to have a multiple choice set of flags and using a many-to-many database relation is massively overkill. Django 1.9 added a builtin modelfield to leverage Postgres’ built-in array support. Unfortunately the default formfield for ArrayField, SimpleArrayField, is not even a little bit useful (it’s a comma-delimited text input).

If you’re writing your own form, you can simply use the MultipleChoiceField formfield, but if you’re using something that builds forms using the ModelForm automagic factories with no overrides (e.g. Wagtail’s admin site), you need a way to specify the formfield.

Instead subclass ArrayField:

from django import forms
from django.contrib.postgres.fields import ArrayField

class ChoiceArrayField(ArrayField):
    A field that allows us to store an array of choices.
    Uses Django 1.9's postgres ArrayField
    and a MultipleChoiceField for its formfield.

    def formfield(self, **kwargs):
        defaults = {
            'form_class': forms.MultipleChoiceField,
            'choices': self.base_field.choices,
        # Skip our parent's formfield implementation completely as we don't
        # care for it.
        # pylint:disable=bad-super-call
        return super(ArrayField, self).formfield(**defaults)

You use this like ArrayField, except that choices is required.

    ('defect', 'Defect'),
    ('enhancement', 'Enhancement'),
flags = ChoiceArrayField(models.CharField(max_length=...,

You can similarly use this with any other field that supports choices, e.g. IntegerField (but you’re not storing choices as integers… are you).


Two new fixtures-related utilities for Django and Wagtail

During constantly bettering today I got around to forking two small tools I wrote from their parent codebases and uploading them to GitHub/PyPI.

wagtailimporter is a Django app that reads Wagtail pages from a yaml file and imports them into the database. It’s designed to handle pages in a much neater way than DB fixtures, including things like foreign key lookups (via Yaml objects) and inline support for StreamField. It’s sort of a work in progress, where I’m adding more support for things as needed.

django_loaddata_stdin is a very small utility that extends Django’s loaddata command to support reading from stdin. This is extremely useful when you have fragments of site config loaded into the database (e.g. Django sites, Allauth socialaccounts), but you don’t want this in a fixture file committed to revision control.

For instance, to deploy my site into Docker, where I don’t have site-config fixture files built into the containers.

docker-compose run web ./ loaddata --format=yaml - << EOF

lazy-loading class-based-views in Django

So one of the nice things with method-based views in Django is the ability to do this sort of thing to load a view at the path frontend.views.home:

urlpatterns = patterns(

    url(r'^$', 'home', name='home'),

Unfortunately, if you’re using class-based-views, you can’t do this:

urlpatterns = patterns(

    url(r'^$', 'HomeView', name='home'),

And instead you had to resort to importing the view and calling HomeView.as_view(). Sort of annoying when you didn’t want to import all of those views.

It turns out however that overloading the code to resolve HomeView is not that difficult, and we can do it with a pretty straightforward monkeypatch. This version uses the kwargs argument of url() to pass keyword arguments to as_view().

from django.conf import urls
from django.views.generic import View

class ClassBasedViewURLPattern(urls.RegexURLPattern):
    A version of RegexURLPattern able to handle class-based-views

    Monkey-patch it in to support class-based-views

    def callback(self):
        Hook locating the view to handle class based views

        view = super(ClassBasedViewURLPattern, self).callback

        if isinstance(view, type) and issubclass(view, View):
            view = view.as_view(**self.default_args)
            self.default_args = {}

        return view

urls.RegexURLPattern = ClassBasedViewURLPattern

Django utility methods (including New Relic deployment notification)

So we’ve moved to Github here at Infoxchange as our primary development platform because pull requests and Travis CI are much nicer than yelling across the room at each other1. To enable Travis to build our code, we’ve needed to move our little utility libraries to Github too. Since some of these were already on pip, it made sense to open source the rest of them too.

The most useful is a package called IXDjango which includes a number of generally useful management commands for Django developers. Especially useful are deploy which will run a sequence of other commands for deployment, and newrelic_notify_deploy which you can use to notify New Relic of your deployment, which annotates all of your graphs with the version number.

We hope these are useful to people.

  1. big shout out to both Github and Travis CI for supporting our not-for-profit mission with gratis private accounts []

More than a side salad: behaviour driven testing and test driven design in Django with Lettuce

Been quiet lately because I am super busy getting a project out of the door. However I did find time to give this talk last night on behaviour driven testing with Lettuce at MelbDjango.

Apologies for the glitches in the PDF. reveal.js is amazing for doing presentations but I didn’t have time to fix the glitches in the PDF output. The presentation is also able to be cloned from Git (view index.html), but you require the fonts (they’re all libre but I was too lazy to use webfonts).

Hopefully, when I get a breather, I can write about some other code I’ve written, or am writing. Or maybe catch up on the rest of my life/projects.