Django

Django is a high-level Python web framework. It has very good documentation at www.djangoproject.com.

• from django.conf import settings
• Check version of Django installation by import django; django.VERSION.
• Return data (not template) from view: return HttpResponse(data)
• Show raw SQL of queryset: print(qs.query)
• Run raw SQL: from django.db import connection; cursor = connection.cursor(); cursor.execute(SQL).
• from django.contrib.auth.models import User as AuthUser
• To get a timezone-aware current time use: from django.utils.timezone import now Midnight is now().replace(hour=0,minute=0,second=0,microsecond=0).

Gotchas

• Fixtures are only looked for in applications with models.py modules.
• HTML 400 Bad Request: When DEBUG is false the ALLOWED_HOSTS check is made. Setting DEBUG to true causes error to disappear!
• ImproperlyConfigured: The included urlconf [project].urls doesn't have any patterns in it: Replace reverse with reverse_lazy. E.g. Class-based view success_url is being evaluated before the URL patterns have been loaded.
• Initially when a ForeignKey is instantiated with a string-based related field, self.rel.to is set to the string-based identifier. It is later converted into the class proper. This can cause an error isinstance() arg 2 must be a class....
  Fix is to ensure that the Model Cache is fully loaded: from django.db.models.loading import cache as model_cache then if not model_cache.loaded: model_cache._populate()

Admin

• Site-wide disable deletion on lists: admin.site.disable_action('delete_selected') in admin.py.
• Specifying blank=False and a default value removes the empty choice from Admin interface. E.g. holiday_calendar = models.ForeignKey(Calendar, blank=False, default='ENG')
• Change List
  • Filter using related data (works even for ManyToMany relationships): list_filter = ['fieldname__relatedfieldname']
• Field
  • date_hierarchy = '[date_field]' Adds a date-based navigation interface for that field.
  • Use filter_horizontal = ('field_name',) to get 2-pane many-to-many widget.
• Inline Admin
  • Remove empty rows: By default Django displays three empty rows in each inline list. To prevent this add extra = 0 to the Inline admin declaration.
  • Remove Delete option: can_delete = False
  • Remove Add button: max_num = 0
• Admin Template
  • Object can be accessed as original.
  • Model as opts.

Fomfield For ForeignKey

The formfield_for_foreignkey method on a ModelAdmin allows you to override the default formfield for a foreign key field. For example, to return a subset of objects for this foreign key field based on the user:

class MyModelAdmin(admin.ModelAdmin):
    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        if db_field.name == "car":
            kwargs["queryset"] = Car.objects.filter(owner=request.user)
        return super(MyModelAdmin, self).formfield_for_foreignkey(db_field, request, **kwargs)

Commands

• python manage.py runserver Run Django development server. On a remote server use python manage.py runserver 0.0.0.0:9000; Project can then be visited using port 9000 on the server's IP.
• python manage.py test [app][.TestCase[.test]] Run unit tests.
• python manage.py shell Start interactive Python shell with Django environment settings. Uses iPython if it is available.
• python manage.py dumpdata --indent 2 --format xml auth.user Create fixture.
• python manage.py help List all available manage commands.
• python manage.py startapp appname Create a new application.

Reset Password using shell

1. python manage.py shell
2. from django.contrib.auth.models import User
3. u=User.objects.get(username__exact='[userid]')
4. u.set_password('[new_password]');
5. u.save()

Forms

• Bound Form: Initial data is ignored if a form is bound. (e.g. form = SampleForm(request.POST)). This means that it is impossible to update field contents in code. Django actually checks that the form.data[fieldname] is immutable.
• Pass error into validated form: form._errors['fieldname'] = ErrorList([u'Error message'])
• Template
  • Display field contents in template (works for ChoiceFields): {{ form.instance.fieldname }}
  • Access field id: {{ field.auto_id }}

Models

• get_or_create looks for an existing entry where all the data matches. If one of these is a numeric then it must be rounded to match the database contents for this processing to work. Also timestamps of rows created in unit tests may differ by 1 second - can cause intermittent problems of attempting to create duplicates.
• Proxy Models must be defined with the same app_label as the base model. Otherwise no permissions are found and access to the proxy is denied.
• If a model has a ForeignKey, it will have access to a Manager that returns all instances of the keyed model. By default, this Manager is named foo_set, where foo is the source model name, lowercased. This Manager returns Querysets.
  If Client model has a many-to-many relationship to User then the following will return all the clients related to a specific user: request.user.client_set.all()
• You can reverse filter on ForeignKeys merely by referring to the other model by lowercase name. E.g. Blog.objects.filter(entry__authors__name='Lennon').

Meta

• Ordering of objects in models.py will be used when listing objects on Admin pages. E.g. ordering = ['something__name', 'something_else__name']
• verbose_name_plural

Natural Keys

A natural key is a tuple of values that can be used to uniquely identify an object instance without using the primary key value. They can be used to serialise data into fixtures to avoid the problem of needing foreign-keyed data to always have the same primary key.

Add a get_by_natural_key function to the model's manager which gets and returns the unique row which matches its parameters. Add a natural_key function to the model to return the tuple of data which uniquely identifies a model instance.

Serialise the data using dumpdata by adding a --natural parameter. python manage.py dumpdata --natural --indent 4 cm > [app]/fixtures/initial_data.json.

Settings

• LOGIN_REDIRECT_URL Default: /accounts/profile/. The URL where requests are redirected after login when the contrib.auth.login view gets no next parameter.

Templates

• Iterate through ManyToManyField contents with .all: {% for keyword in item.keywords.all %}{{ keyword }} {% endfor %}
• TEMPLATE_STRING_IF_INVALID = 'Missing variable %s' Display missing variables in templates.
• Tags
• {{ value|date:"[format]" }} Format as date.
• {{ value|join:", " }} Will output the items in list value separated by ", ".
• |safe, |escape and |escapejs templatetags control how values are added to templates. |escapejs Escapes characters for use in Javascript strings.
• Don't put comma after last item: {% if forloop.last %}{% else %},{% endif %}
• Return data (not template) from view: return HttpResponse(data)
• from django.utils.html import strip_tags Remove html tags from text.

Iterate through field choices

{% for id, label in form.banana.field.choices %}
    {{ id }}': '{{ label }}'{% if forloop.last %}{% else %},{% endif %}
{% endfor %}

Views

• To return no data to a request use return HttpResponse('')
• Return data (not template) from view: return HttpResponse(data)
• User getlist() to retrieve list from request.GET. E.g. search_engine_ids = request.GET.getlist('search_engines', [])
• Use from django.conf import settings to access settings. This contains the application settings at runtime.
• Show SQL which will be run by adding query.__str__() to queryset.
• Return data (not template) from view: return HttpResponse(data)
• Redirects: from django.http import HttpResponseRedirect then return HttpResponseRedirect('/path/')
• Add help = __doc__ to commands to reuse the docstring.
• @staff_member_required: Decorator to ensure user is logged in and has is_staff set before allowing them access to a view. Import: from django.contrib.admin.views.decorators import staff_member_required