==================
Tips and Solutions
==================

Common problems for declared filters
------------------------------------

Below are some of the common problems that occur when declaring filters. It is
recommended that you read this as it provides a more complete understanding of
how filters work.


Filter ``field_name`` and ``lookup_expr`` not configured
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While ``field_name`` and ``lookup_expr`` are optional, it is recommended that you specify
them. By default, if ``field_name`` is not specified, the filter's name on the
``FilterSet`` class will be used. Additionally, ``lookup_expr`` defaults to
``exact``. The following is an example of a misconfigured price filter:

.. code-block:: python

    class ProductFilter(django_filters.FilterSet):
        price__gt = django_filters.NumberFilter()

The filter instance will have a field name of ``price__gt`` and an ``exact``
lookup type. Under the hood, this will incorrectly be resolved as:

.. code-block:: python

    Product.objects.filter(price__gt__exact=value)

The above will most likely generate a ``FieldError``. The correct configuration
would be:

.. code-block:: python

    class ProductFilter(django_filters.FilterSet):
        price__gt = django_filters.NumberFilter(field_name='price', lookup_expr='gt')


Missing ``lookup_expr`` for text search filters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's quite common to forget to set the lookup expression for :code:`CharField`
and :code:`TextField` and wonder why a search for "foo" does not return results
for "foobar". This is because the default lookup type is ``exact``, but you
probably want to perform  an ``icontains`` lookup.


Filter and lookup expression mismatch (in, range, isnull)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's not always appropriate to directly match a filter to its model field's
type, as some lookups expect different types of values. This is a commonly
found issue with ``in``, ``range``, and ``isnull`` lookups. Let's look
at the following product model:

.. code-block:: python

    class Product(models.Model):
        category = models.ForeignKey(Category, null=True)

Given that ``category`` is optional, it's reasonable to want to enable a search
for uncategorized products. The following is an incorrectly configured
``isnull`` filter:

.. code-block:: python

    class ProductFilter(django_filters.FilterSet):
        uncategorized = django_filters.NumberFilter(field_name='category', lookup_expr='isnull')

So what's the issue? While the underlying column type for ``category`` is an
integer, ``isnull`` lookups expect a boolean value. A ``NumberFilter`` however
only validates numbers. Filters are not `'expression aware'` and won't change
behavior based on their ``lookup_expr``. You should use filters that match the
data type of the lookup expression `instead` of the data type underlying the
model field. The following would correctly allow you to search for both
uncategorized products and products for a set of categories:

.. code-block:: python

    class NumberInFilter(django_filters.BaseInFilter, django_filters.NumberFilter):
        pass

    class ProductFilter(django_filters.FilterSet):
        categories = NumberInFilter(field_name='category', lookup_expr='in')
        uncategorized = django_filters.BooleanFilter(field_name='category', lookup_expr='isnull')

More info on constructing ``in`` and ``range`` csv :ref:`filters <base-in-filter>`.


Filtering by empty values
-------------------------

There are a number of cases where you may need to filter by empty or null
values. The following are some common solutions to these problems:


Filtering by null values
~~~~~~~~~~~~~~~~~~~~~~~~

As explained in the above "Filter and lookup expression mismatch" section, a
common problem is how to correctly filter by null values on a field.

Solution 1: Using a ``BooleanFilter`` with ``isnull``
"""""""""""""""""""""""""""""""""""""""""""""""""""""

Using ``BooleanFilter`` with an ``isnull`` lookup is a builtin solution used by
the FilterSet's automatic filter generation. To do this manually, simply add:

.. code-block:: python

    class ProductFilter(django_filters.FilterSet):
        uncategorized = django_filters.BooleanFilter(field_name='category', lookup_expr='isnull')

.. note::

    Remember that the filter class is validating the input value. The underlying
    type of the mode field is not relevant here.

You may also reverse the logic with the ``exclude`` parameter.

.. code-block:: python

    class ProductFilter(django_filters.FilterSet):
        has_category = django_filters.BooleanFilter(field_name='category', lookup_expr='isnull', exclude=True)

Solution 2: Using ``ChoiceFilter``'s null choice
""""""""""""""""""""""""""""""""""""""""""""""""

If you're using a ChoiceFilter, you may also filter by null values by enabling
the ``null_label`` parameter. More details in the ``ChoiceFilter`` reference
:ref:`docs <choice-filter>`.

.. code-block:: python

    class ProductFilter(django_filters.FilterSet):
        category = django_filters.ModelChoiceFilter(
            field_name='category', lookup_expr='isnull',
            null_label='Uncategorized',
            queryset=Category.objects.all(),
        )

Solution 3: Combining fields w/ ``MultiValueField``
"""""""""""""""""""""""""""""""""""""""""""""""""""

An alternative approach is to use Django's ``MultiValueField`` to manually add
in a ``BooleanField`` to handle null values. Proof of concept:
https://github.com/carltongibson/django-filter/issues/446


Filtering by an empty string
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

It's not currently possible to filter by an empty string, since empty values are
interpreted as a skipped filter.

    GET http://localhost/api/my-model?myfield=

Solution 1: Magic values
""""""""""""""""""""""""

You can override the ``filter()`` method of a filter class to specifically check
for magic values. This is similar to the ``ChoiceFilter``'s null value handling.

    GET http://localhost/api/my-model?myfield=EMPTY

.. code-block:: python

    class MyCharFilter(filters.CharFilter):
        empty_value = 'EMPTY'

        def filter(self, qs, value):
            if value != self.empty_value:
                return super().filter(qs, value)

            qs = self.get_method(qs)(**{'%s__%s' % (self.field_name, self.lookup_expr): ""})
            return qs.distinct() if self.distinct else qs


Solution 2: Empty string filter
"""""""""""""""""""""""""""""""

It would also be possible to create an empty value filter that exhibits the same
behavior as an ``isnull`` filter.

    GET http://localhost/api/my-model?myfield__isempty=false

.. code-block:: python

    from django.core.validators import EMPTY_VALUES

    class EmptyStringFilter(filters.BooleanFilter):
        def filter(self, qs, value):
            if value in EMPTY_VALUES:
                return qs

            exclude = self.exclude ^ (value is False)
            method = qs.exclude if exclude else qs.filter

            return method(**{self.field_name: ""})


    class MyFilterSet(filters.FilterSet):
        myfield__isempty = EmptyStringFilter(field_name='myfield')

        class Meta:
            model = MyModel
            fields = []


Filtering by relative times
---------------------------

Given a model with a timestamp field, it may be useful to filter based on relative times.
For instance, perhaps we want to get data from the past *n* hours.
This could be accomplished the with a ``NumberFilter`` that invokes a custom method.

.. code-block:: python

    from django.utils import timezone
    from datetime import timedelta
    ...

    class DataModel(models.Model):
        time_stamp = models.DateTimeField()


    class DataFilter(django_filters.FilterSet):
        hours = django_filters.NumberFilter(
            field_name='time_stamp', method='get_past_n_hours', label="Past n hours")

        def get_past_n_hours(self, queryset, field_name, value):
            time_threshold = timezone.now() - timedelta(hours=int(value))
            return queryset.filter(time_stamp__gte=time_threshold)

        class Meta:
            model = DataModel
            fields = ('hours',)

Using ``initial`` values as defaults
------------------------------------

In pre-1.0 versions of django-filter, a filter field's ``initial`` value was used as a
default when no value was submitted. This behavior was not officially supported and has
since been removed.


.. warning:: It is recommended that you do **NOT** implement the below as it adversely
    affects usability. Django forms don't provide this behavior for a reason.

    - Using initial values as defaults is inconsistent with the behavior of Django forms.
    - Default values prevent users from filtering by empty values.
    - Default values prevent users from skipping that filter.

If defaults are necessary though, the following should mimic the pre-1.0 behavior:

.. code-block:: python

    class BaseFilterSet(FilterSet):

        def __init__(self, data=None, *args, **kwargs):
            # if filterset is bound, use initial values as defaults
            if data is not None:
                # get a mutable copy of the QueryDict
                data = data.copy()

                for name, f in self.base_filters.items():
                    initial = f.extra.get('initial')

                    # filter param is either missing or empty, use initial as default
                    if not data.get(name) and initial:
                        data[name] = initial

            super().__init__(data, *args, **kwargs)


Adding model field ``help_text`` to filters
-------------------------------------------

Model field ``help_text`` is not used by filters by default. It can be added
using a simple FilterSet base class::

  class HelpfulFilterSet(django_filters.FilterSet):
      @classmethod
      def filter_for_field(cls, f, name, lookup_expr):
          filter = super(HelpfulFilterSet, cls).filter_for_field(f, name, lookup_expr)
          filter.extra['help_text'] = f.help_text
          return filter