Before We Start

The List View

The main components of Django admin list view:

Example admin list view including some of the components discussed in the article

Logging

Most of Django’s work is performing SQL queries so our main focus will be on minimizing the amount of queries. To keep track of query execution you can use one of the following:

django-debug-toolbar — Very nice utility that adds a little panel on the side of the screen with a list of SQL queries executed and other useful metrics.

If you don’t like dependencies (like us) you can log SQL queries to the console by adding the following logger in settings.py:

LOGGING = {

...

'loggers': {

'django.db.backends': {

'level': 'DEBUG',

},

},

...

}

The N+1 Problem

The N+1 problem is a well known problem in ORMs. To illustrate the problem let’s say we have this schema:

class Category(models.Model):

name = models.CharField(max_length=50) def__str__(self):

return self.name class Product(models.Model):

name = models.CharField(max_length=50)

category = models.ForeignKey(Category)

By implementing __str__ we tell Django that we want the name of the category to be used as the default description of the object. Whenever we print a category object Django will fetch the name of the category.

A simple admin page for our Product model might look like this:

@admin.register(models.Product)

class ProductAdmin(admin.ModelAdmin):

list_display = (

'id',

'name',

'category',

)

This seems innocent enough but the SQL log reveals the horror:

(0.000) SELECT COUNT(*) AS “__count” FROM “app_product”; args=()

(0.002) SELECT “app_product”.”id”, “app_product”.”name”, "app_product"."category_id" FROM “app_product” ORDER BY “app_product”.”id” DESC LIMIT 100; args=()

(0.000) SELECT “app_category”.”id”, “app_category”.”name” FROM “app_category” where “app_category”.”id” = 1; args=(1)

(0.000) SELECT “app_category”.”id”, “app_category”.”name” FROM “app_category” where “app_category”.”id” = 2; args=(2)

(0.000) SELECT “app_category”.”id”, “app_category”.”name” FROM “app_category” where “app_category”.”id” = 1; args=(1)

(0.000) SELECT “app_category”.”id”, “app_category”.”name” FROM “app_category” where “app_category”.”id” = 4; args=(4)

(0.000) SELECT “app_category”.”id”, “app_category”.”name” FROM “app_category” where “app_category”.”id” = 3; args=(3) ... (0.000) SELECT “app_category”.”id”, “app_category”.”name” FROM “app_category” where “app_category”.”id” = 2; args=(2)

(0.000) SELECT “app_category”.”id”, “app_category”.”name” FROM “app_category” where “app_category”.”id” = 99; args=(99)

(0.000) SELECT “app_category”.”id”, “app_category”.”name” FROM “app_category” where “app_category”.”id” = 104; args=(104)

Django first counts the objects (more on that later), then fetches the actual objects (limiting to the default page size of 100) and then passes the data on to the template for rendering. We used the category name as the description of the Category object so for each product Django has to fetch the category name — this results in 100 additional queries.

To tell Django we want to perform a join instead of fetching the names of the categories one by one,we can use list_select_related

@admin.register(models.Product)

class ProductAdmin(admin.ModelAdmin):

list_display = (

'id',

'name',

'category',

) list_select_related = (

'category',

)

Now the SQL log looks much nicer. Instead of 101 queries we have only 1:

(0.004) SELECT “app_product”.”id”, “app_product”.”name”, “app_product”.”category_id”, “app_category”.”id”, “app_category”.”name” FROM “app_product” INNER JOIN “app_category” on (“app_product”.”category_id” = “app_category”.”id”) ORDER BY “app_product”.”id” DESC LIMIT 100; args=()

To understand the real impact of this setting consider the following — Django default page size is 100 objects. If you have one related fields you have ~101 queries, if you have two related objects displayed in the list view you have ~201 queries and so on.

Fetching related fields in a join can only work for ForeignKey relations. If you wish to display ManyToMany relations it’s a bit more complicated (and most of the time wrong, but keep reading).

Related Fields

Sometimes it can be useful to quickly navigate between objects. After trying for a while to teach support personnel to filter using URL parameters we finally gave up and created two simple decorators.

admin_link

Create a link to a detail page of a related model:

def admin_change_url(obj):

app_label = obj._meta.app_label

model_name = obj._meta.model.__name__.lower()

return reverse('admin:{}_{}_change'.format(

app_label, model_name

), args=(obj.pk,)) def admin_link(attr, short_description, empty_description="-"):

"""Decorator used for rendering a link to a related model in

the admin detail page. attr (str):

Name of the related field.

short_description (str):

Name if the field.

empty_description (str):

Value to display if the related field is None. The wrapped method receives the related object and should

return the link text. Usage:

@admin_link('credit_card', _('Credit Card'))

def credit_card_link(self, credit_card):

return credit_card.name

"""

def wrap(func):

def field_func(self, obj):

related_obj = getattr(obj, attr)

if related_obj is None:

return empty_description

url = admin_change_url(related_obj)

return format_html(

'<a href="{}">{}</a>',

url,

func(self, related_obj)

)

field_func.short_description = short_description

field_func.allow_tags = True

return field_func

return wrap

The decorator will render a link (a) to the related model in both the list view and the detail view. In our case we might want to add a link from each product to its category detail page in both the list view and the detail view:

@admin.register(models.Product)

class ProductAdmin(admin.ModelAdmin):

list_display = (

'id',

'name',

'category_link',

) admin_select_related = (

'category'

) @admin_link('category', _('Category'))

def category_link(self, category):

return category

admin_changelist_link

For more complicated links such as “all the products of a category” we created a decorator that accepts a query string and link to the list view of a related model:

def admin_changelist_url(model):

app_label = model._meta.app_label

model_name = model.__name__.lower()

return reverse('admin:{}_{}_changelist'.format(

app_label,

model_name)

) def admin_changelist_link(

attr,

short_description,

empty_description="-",

query_string=None

):

"""Decorator used for rendering a link to the list display of

a related model in the admin detail page. attr (str):

Name of the related field.

short_description (str):

Field display name.

empty_description (str):

Value to display if the related field is None.

query_string (function):

Optional callback for adding a query string to the link.

Receives the object and should return a query string. The wrapped method receives the related object and

should return the link text.



def credit_card_link(self, credit_card):

return credit_card.name

"""

def wrap(func):

def field_func(self, obj):

related_obj = getattr(obj, attr)

if related_obj is None:

return empty_description

url = admin_changelist_url(related_obj.model)

if query_string:

url += '?' + query_string(obj)

return format_html(

'<a href="{}">{}</a>',

url,

func(self, related_obj)

)

field_func.short_description = short_description

field_func.allow_tags = True

return field_func

return wrap Usage: @admin_changelist_lin k('credit_card', _('Credit Card'))def credit_card_link(self, credit_card):return credit_card.name"""def wrap(func):def field_func(self, obj):related_obj = getattr(obj, attr)if related_obj is None:return empty_descriptionurl = admin_changelist_url(related_obj.model)if query_string:url += '?' + query_string(obj)return format_html('{}',url,func(self, related_obj)field_func.short_description = short_descriptionfield_func.allow_tags = Truereturn field_funcreturn wrap

Adding a link from a category to a list of it’s products, the Category admin looks like this:

@admin.register(models.Category)

class CategoryAdmin(admin.ModelAdmin):

list_display = (

'id',

'name',

'products_link',

) @admin_changelist_link(

'products',

_('Products'),

query_string=lambda c: 'category_id={}'.format(c.pk)

)

def products_link(self, products):

return _('Products')

Be careful with the products argument — it’s very tempting to do something like return ‘see {} products’.format(products.count()) but it will result in additional queries.

In the detail page Django creates an editable element for each field. Text and numeric fields will be rendered as regular input field. Choice fields and foreign key fields will be rendered as a <select> element. Rendering a select box requires Django to do the following:

Fetch the options — the entire related model + descriptions (remember the N+1 problem?). Render the option list — one option for each related model instance.

One common example that is often overlooked is foreign key to the User model. When you have 100 users you might not feel the load but what happens when you suddenly have 100K users? The detail page will fetch the entire users table and the option list will make the resulting HTML file huge. We pay twice — first for the full table scan and then for downloading the html file (not to mention the memory required to generate the file).

Having a select box with 100K options is not really usable. The easiest way to prevent Django from rendering a field as a <select> element is to make it a read only field in the admin:

@admin.register(SomeModel)

def SomeModelAdmin(admin.ModelAdmin):

readonly_fields = (

'user',

)

This will render the description of the related model without being able to change it in the admin.

Filters

As mentioned above, we often use the admin interface as a day to day tool for general support. We found that most of the times we use the same filters — only active users, users registered in the last month, successful transactions, etc. Once we realized that, we asked ourselves, why fetch the entire dataset if we are most likely to immediately apply a filter. We started to look for a way to apply a default filter when entering the model list view. Other than the usability perk it also makes paging cheaper.

DefaultFilterMixin

There are many approaches for applying a default filter. Some of them involve fancy custom filters and some inject magic query parameters to the request.

We found this approach to be simple and straightforward:

from urllib.parse import urlencode

from django.shortcuts import redirect class DefaultFilterMixin:

def get_default_filters(self, request):

"""Set default filters to the page. request (Request) Returns (dict):

Default filter to encode.

"""

raise NotImplementedError() def changelist_view(self, request, extra_context=None):

ref = request.META.get('HTTP_REFERER', '')

path = request.META.get('PATH_INFO', '') # If already have query parameters or if the page

# was referred from it self (by drilldown or redirect)

# don't apply default filter.

if request.GET or ref.endswith(path):

return super().changelist_view(

request,

extra_context=extra_context

) query = urlencode(self.get_default_filters(request))

return redirect('{}?{}'.format(path, query))

If the list view was accessed from a different view and no query params were specified we generate a default query and redirect.

Let’s add a default filter to our Product page to filter only products created in the last month:

from django.utils import timezone @admin.register(models.Product)

class ProductAdmin(DefaultFilterMixin, admin.ModelAdmin):

date_hierarchy = 'created' def get_default_filters(self, request):

now = timezone.now()

return {

'created__year': now.year,

'created__month': now.month,

}

If we drill down from within the page or if we get to the page with query parameters (using admin_changelist_link for example) the default filter will not be applied.

Profit!

Quick Bits

Some neat tricks we gathered over time:

Prevent Django from displaying the total amount of rows in the list view. Setting this option to False saves a count(*) query on the queryset.

When performing a query the entire result set is put into memory for processing. If you have large columns in your model (such as JSON or Text fields) it might be a good idea to defer them until you really need to use them and save some memory. To defer fields override get_queryset (or better yet, create a mixin!).

Change the admin default URL route

This is definitely not the only precaution you should take to protect your admin page but it can make it harder for “curious” users to reach the login page. In your main urls.py override the default admin route:

from django.conf.urls import include, url

from django.contrib import admin urlpatterns = [

url(r'^foo/', include(admin.site.urls)),

]

We found this index can improve query when filtering on the date_hierarchy (PostgresSQL 9.4):

CREATE INDEX yourmodel_date_hierarchy_ix ON yourmodel_table (

extract('day' from created at time zone 'America/New_York'),

extract('month' from created at time zone 'America/New_York'),

extract('year' from created at time zone 'America/New_York')

);

Make sure to change table name, index name, the date hierarchy column and the time zone.

Conclusion

Even if you don’t have 100K users and millions of records in the database it is still important to keep the admin tidy. Bad code has this nasty tendency of biting you in the ass when you least expect it.