Using django-filter

Django-filter provides a simple way to filter down a queryset based on parameters a user provides. Say we have a Product model and we want to let our users filter which products they see on a list page.

The model

Let’s start with our model:

from django.db import models

class Product(models.Model):
    name = models.CharField(max_length=255)
    price = models.DecimalField()
    description = models.TextField()
    release_date = models.DateField()
    manufacturer = models.ForeignKey(Manufacturer)

The filter

We have a number of fields and we want to let our users filter based on the price or the release_date. We create a FilterSet for this:

import django_filters

class ProductFilter(django_filters.FilterSet):
    class Meta:
        model = Product
        fields = ['price', 'release_date']

As you can see this uses a very similar API to Django’s ModelForm. Just like with a ModelForm we can also override filters, or add new ones using a declarative syntax:

import django_filters

class ProductFilter(django_filters.FilterSet):
    price = django_filters.NumberFilter(lookup_type='lt')
    class Meta:
        model = Product
        fields = ['price', 'release_date']

Filters take a lookup_type argument which specifies what lookup type to use with Django’s ORM. So here when a user entered a price it would show all Products with a price less than that.

It’s quite common to forget to set lookup type for `CharField`s/`TextField`s and wonder why search for “foo” doesn’t return result for “foobar”. It’s because default lookup type is exact text, but you probably want `icontains` lookup field.

The FilterSet Meta class fields can additionally be set using a Dictionary to specify multiple lookup_type filters without significant code duplication:

import django_filters

class ProductFilter(django_filters.FilterSet):
    class Meta:
        model = Product
        fields = {'price': ['lt', 'gt'],
                  'release_date': ['exact'],
                 }

The above would generate ‘price__lt’, ‘price__gt’ and ‘release_date’ filters. The filter lookup type keyword ‘exact’ is the default and therefore never added to a filter name.

Items in the fields sequence in the Meta class may include “relationship paths” using Django’s __ syntax to filter on fields on a related model:

class ProductFilter(django_filters.FilterSet):
    class Meta:
        model = Product
        fields = ['manufacturer__country']

Filters also take any arbitrary keyword arguments which get passed onto the django.forms.Field initializer. These extra keyword arguments get stored in Filter.extra, so it’s possible to override the initializer of a FilterSet to add extra ones:

class ProductFilter(django_filters.FilterSet):
    class Meta:
        model = Product
        fields = ['manufacturer']

    def __init__(self, *args, **kwargs):
        super(ProductFilter, self).__init__(*args, **kwargs)
        self.filters['manufacturer'].extra.update(
            {'empty_label': 'All Manufacturers'})

Like django.contrib.admin.ModelAdmin, it is possible to override default filters for all the models fields of the same kind using filter_overrides:

class ProductFilter(django_filters.FilterSet):
    filter_overrides = {
        models.CharField: {
            'filter_class': django_filters.CharFilter,
            'extra': lambda f: {
                'lookup_type': 'icontains',
            }
        }
    }

    class Meta:
        model = Product
        fields = ['name']

MethodFilter

If you want fine control over each individual filter attribute, you can use the MethodFilter filter.

By passing in the name of a custom defined filter function as an action, the filter attribute gets linked to the custom filter function. Here is an example of overriding the filter function of the filter attribute username

class F(django_filters.FilterSet):
        username = MethodFilter(action='my_custom_filter')

        class Meta:
            model = User
            fields = ['username']

        def my_custom_filter(self, queryset, value):
            return queryset.filter(
                username=value
            )

The filter function can also be defined outside of the filter class scope. Though you would need to pass in the actual function value, not it’s name.

def my_custom_filter(queryset, value):
    return queryset.filter(
        username=value
    )

class F(django_filters.FilterSet):
    # Notice: In this case, action accepts a func, not a string
    username = MethodFilter(action=filter_username)

    class Meta:
        model = User
        fields = ['username']

Lastly, when using a MethodFilter, there is no need to define an action. You may simply do the following and filter_username will be auto-detected and used.

class F(FilterSet):
    username = MethodFilter()

    class Meta:
        model = User
        fields = ['username']

    def filter_username(self, queryset, value):
        return queryset.filter(
            username__contains='ke'
        )

Under the hood, if action not is defined, django_filter searches for a class method with a name that follows the pattern filter_{{ATTRIBUTE_NAME}}. For example, if the attribute name is email, then the filter class will be scanned for the filter function filter_email. If no action is provided, and no filter class function is found, then the filter attribute will be left unfiltered.

The view

Now we need to write a view:

def product_list(request):
    f = ProductFilter(request.GET, queryset=Product.objects.all())
    return render_to_response('my_app/template.html', {'filter': f})

If a queryset argument isn’t provided then all the items in the default manager of the model will be used.

If you want to access the filtered objects in your views, for example if you want to paginate them, you can do that. They are in f.qs

The URL conf

We need a URL pattern to call the view:

url(r'^list$', views.product_list)

The template

And lastly we need a template:

{% extends "base.html" %}

{% block content %}
    <form action="" method="get">
        {{ filter.form.as_p }}
        <input type="submit" />
    </form>
    {% for obj in filter %}
        {{ obj.name }} - ${{ obj.price }}<br />
    {% endfor %}
{% endblock %}

And that’s all there is to it! The form attribute contains a normal Django form, and when we iterate over the FilterSet we get the objects in the resulting queryset.

Other Meta options

Ordering using order_by

You can allow the user to control ordering by providing the order_by argument in the Filter’s Meta class. order_by can be either a list or tuple of field names, in which case those are the options, or it can be a bool which, if True, indicates that all fields that the user can filter on can also be sorted on. An example or ordering using a list:

import django_filters

class ProductFilter(django_filters.FilterSet):
    price = django_filters.NumberFilter(lookup_type='lt')
    class Meta:
        model = Product
        fields = ['price', 'release_date']
        order_by = ['price']

If you want to control the display of items in order_by, you can set it to a list or tuple of 2-tuples in the format (field_name, display_name). This lets you override the displayed names for your ordering fields:

order_by = (
    ('name', 'Company Name'),
    ('average_rating', 'Stars'),
)

Note that the default query parameter name used for ordering is o. You can override this by setting an order_by_field attribute on the FilterSet class to the string value you would like to use.

Custom Forms using form

The inner Meta class also takes an optional form argument. This is a form class from which FilterSet.form will subclass. This works similar to the form option on a ModelAdmin.

Group fields with together

The inner Meta class also takes an optional together argument. This is a list of lists, each containing field names. For convenience can be a single list/tuple when dealing with a single set of fields. Fields within a field set must either be all or none present in the request for FilterSet.form to be valid:

import django_filters

class ProductFilter(django_filters.FilterSet):
    class Meta:
        model = Product
        fields = ['price', 'release_date', 'rating']
        together = ['rating', 'price']

Non-Meta options

Note that these options do not go in the Meta class, they are specified directly in your FilterSet class.

strict

The strict option controls whether results are returned when an invalid value is specified by the user for any filter field. By default, strict is set to STRICTNESS.RETURN_NO_RESULTS meaning that an empty queryset is returned if any field contains an invalid value. You can loosen this behavior by setting strict to STRICTNESS.IGNORE which will effectively ignore a filter field if its value is invalid. A third option of STRICTNESS.RAISE_VALIDATION_ERROR will cause a ``ValidationError` to be raised if any field contains an invalid value.

Overriding FilterSet methods

get_ordering_field()

If you want to use a custom widget, or in any other way override the ordering field you can override the get_ordering_field() method on a FilterSet. This method just needs to return a Form Field.

Ordering on multiple fields, or other complex orderings can be achieved by overriding the FilterSet.get_order_by() method. This is passed the selected order_by value, and is expected to return an iterable of values to pass to QuerySet.order_by. For example, to sort a User table by last name, then first name:

class UserFilter(django_filters.FilterSet):
    class Meta:
        order_by = (
            ('username', 'Username'),
            ('last_name', 'Last Name')
        )

    def get_order_by(self, order_value):
        if order_value == 'last_name':
            return ['last_name', 'first_name']
        return super(UserFilter, self).get_order_by(order_value)

Generic View

In addition to the above usage there is also a class-based generic view included in django-filter, which lives at django_filters.views.FilterView. You must provide either a model or filterset_class argument, similar to ListView in Django itself:

# urls.py
from django.conf.urls import patterns, url
from django_filters.views import FilterView
from myapp.models import Product

urlpatterns = patterns('',
    (r'^list/$', FilterView.as_view(model=Product)),
)

You must provide a template at <app>/<model>_filter.html which gets the context parameter filter. Additionally, the context will contain object_list which holds the filtered queryset.

A legacy functional generic view is still included in django-filter, although its use is deprecated. It can be found at django_filters.views.object_filter. You must provide the same arguments to it as the class based view:

# urls.py
from django.conf.urls import patterns, url
from myapp.models import Product

urlpatterns = patterns('',
    (r'^list/$', 'django_filters.views.object_filter', {'model': Product}),
)

The needed template and its context variables will also be the same as the class-based view above.