Django-Idioms.md

Django Idioms

General Tips

• put methods that apply to a single instance of a Model on the model class itself
• put methods meant to query against a Models entire table that models corresponding managers.py file.
• if a models.py file gets too long to manage then create a models directory and place an __init__ file in it.
• try to avoid using signals, but if you must then put them in a signals.py file

when to use null=True vs blank=True

class MyModel(models.Model):
    a = models.DateTimeField(null=True)  # blank fields get stored as NULL in your DB column
    b = models.DateTimeField(blank=True) # determines if field will be required in forms

Alwasy be specific in the type of error you want to catch in your try/catch statements, AKA errors should never pass silently unless explicitly silenced.

# this is BAD
try:
    x = MyModel.objects.all()
except Exception:
    pass

# this is GOOD
try:
    x = MyModel.objects.all()
except MyModel.DoesNotExist:
    # do something ...
    

---

Django ORM Tips

exists()

.exists() is used to check if the queryset returned anything.

iterator()

.iterator() should always be used when you want to iterate over the results of a queryset only once.

select_related()

.select_related() For use in forward ForeignKey, forward OneToOne, and backward OneToOne relationships.

prefetch_related()

.prefetch_related Performs a separate lookup for each relationship and does the 'joining' in Python. For use with ManyToMany, ManyToOne, GenericRelations, and GenericForeignKey relations only.

Note - this gets ignored if you're using the .iterator() method

defer()

.defer() if you're using the results of a queryset in a situation where you might not need particular fields for the initial fetch of data, then you can pass the names of a fields you don't immediately need (aka you can defer them). This saves the expensive processing power spent converting the fields to Python objects.

NoodleUsers.object.defer("joined_date")

bulk_create()

.bulk_create is used to efficently create multiple items at once.

Question.object.bulk_create([
	Question(question='what'),
	Question(question='where'),
	Question(question='when'),
])

get_or_create()

.get_or_create() is a useful shortcut method

# replaces this
try:
	u = Person.objects.get(name='bob')
except Person.DoesNotExist:
	p = Person(name='bob')
	p.save()

# with this instead:
p, create = Person.objects.get_or_create(name='bob')

---

Noodle 3rd Party Libraries

Use Django-Model-Utils TimeStampedModel when you want to add a created and modified datetime field to a model.

from model_utils.models import TimeStampedModel

class YourModel(TimeStampedModel):
   	pass

Use Django-Braces Access Mixins when you want to add mixins to your views that restrict access to certain views.

For example, restrict a view to superuser use only you would do:

from django.views import TemplateView
from braces.views import SuperuserRequiredMixin

class SomeView(SuperuserRequiredMixin, TemplateView):
    template_name = u"path/to/template.html"

Django-Extensions

• Why you should use runserver_plus instead of builtin runserver
• Why you should use shell_plus instead of builtin shell
• When the sqldiff command is useful, etc

---

Templating Conventions

Try to stick to MVC standards by keeping as much business logic out of templates as you can.

Try to avoid processing large datasets in templates. For example:

<!--this is bad-->
{% for usr in NoodleUsers %}
    {% if usr.is_staff %} ... {% endif %}
{% endfor %}

Make use of the Django-Absolute template tag when creating anchor tags:

<a href='{% absolute "index" %}'>link</a>

---

Unit Tests

• mock library
• make as few database calls as you can, they are huge bottleneck
• Avoid using fixtures when possible. Use FactoryBoy instead.

---

Debugging

• how to use ipdb
• how to use logging and where to find logs
• how to use Django-Extensions and Django-Debug-Toolbar

---

South Database Migrations

things to know about South

TODO: explain some of these commands (note dj is an alias for python mange.py

>>> dj migrate --help

>>> dj migrate --list

>>> dj schemamigration <app-name> --initial

>>> dj migrate <app-name>

>>> dj schemamigration <app-name> --auto

>>> dj convert_to_south <app-name>

>>> dj migrate <app-name> <migration-number> --fake

>>> dj migrate --ignore-ghost-migrations

>>> dj migrate --delete-ghost-migrations

>>> dj migrate <app-name> <migration-number> --fake --delete-ghost-migrations