216 lines
7.1 KiB
Plaintext
216 lines
7.1 KiB
Plaintext
==================
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Tips and Solutions
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==================
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Common problems for declared filters
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------------------------------------
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Below are some of the common problem that occur when declaring filters. It is
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recommended that you read this as it provides a more complete understanding of
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how filters work.
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Filter ``name`` and ``lookup_expr`` not configured
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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While ``name`` and ``lookup_expr`` are optional, it is recommended that you specify
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them. By default, if ``name`` is not specified, the filter's name on the
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filterset class will be used. Additionally, ``lookup_expr`` defaults to
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``exact``. The following is an example of a misconfigured price filter:
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.. code-block:: python
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class ProductFilter(django_filters.FilterSet):
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price__gt = django_filters.NumberFilter()
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The filter instance will have a field name of ``price__gt`` and an ``exact``
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lookup type. Under the hood, this will incorrectly be resolved as:
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.. code-block:: python
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Produce.objects.filter(price__gt__exact=value)
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The above will most likely generate a ``FieldError``. The correct configuration
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would be:
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.. code-block:: python
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class ProductFilter(django_filters.FilterSet):
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price__gt = django_filters.NumberFilter(name='price', lookup_expr='gt')
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Missing ``lookup_expr`` for text search filters
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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It's quite common to forget to set the lookup expression for :code:`CharField`
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and :code:`TextField` and wonder why a search for "foo" does not return results
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for "foobar". This is because the default lookup type is ``exact``, but you
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probably want to perform an ``icontains`` lookup.
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Filter and lookup expression mismatch (in, range, isnull)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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It's not always appropriate to directly match a filter to its model field's
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type, as some lookups expect different types of values. This is a commonly
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found issue with ``in``, ``range``, and ``isnull`` lookups. Let's look
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at the following product model:
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.. code-block:: python
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class Product(models.Model):
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category = models.ForeignKey(Category, null=True)
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Given that ``category`` is optional, it's reasonable to want to enable a search
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for uncategorized products. The following is an incorrectly configured
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``isnull`` filter:
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.. code-block:: python
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class ProductFilter(django_filters.FilterSet):
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uncategorized = django_filters.NumberFilter(name='category', lookup_expr='isnull')
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So what's the issue? While the underlying column type for ``category`` is an
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integer, ``isnull`` lookups expect a boolean value. A ``NumberFilter`` however
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only validates numbers. Filters are not `'expression aware'` and won't change
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behavior based on their ``lookup_expr``. You should use filters that match the
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data type of the lookup expression `instead` of the data type underlying the
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model field. The following would correctly allow you to search for both
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uncategorized products and products for a set of categories:
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.. code-block:: python
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class NumberInFilter(django_filters.BaseInFilter, django_filters.NumberFilter):
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pass
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class ProductFilter(django_filters.FilterSet):
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categories = NumberInFilter(name='category', lookup_expr='in')
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uncategorized = django_filters.BooleanFilter(name='category', lookup_expr='isnull')
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More info on constructing ``in`` and ``range`` csv :ref:`filters <base-in-filter>`.
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Filtering by empty values
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-------------------------
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There are a number of cases where you may need to filter by empty or null
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values. The following are some common solutions to these problems:
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Filtering by null values
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~~~~~~~~~~~~~~~~~~~~~~~~
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As explained in the above "Filter and lookup expression mismatch" section, a
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common problem is how to correctly filter by null values on a field.
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Solution 1: Using a ``BooleanFilter`` with ``isnull``
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"""""""""""""""""""""""""""""""""""""""""""""""""""""
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Using ``BooleanFilter`` with an ``isnull`` lookup is a builtin solution used by
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the FilterSet's automatic filter generation. To do this manually, simply add:
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.. code-block:: python
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class ProductFilter(django_filters.FilterSet):
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uncategorized = django_filters.BooleanFilter(name='category', lookup_expr='isnull')
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.. note::
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Remember that the filter class is validating the input value. The underlying
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type of the mode field is not relevant here.
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You may also reverse the logic with the ``exclude`` parameter.
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.. code-block:: python
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class ProductFilter(django_filters.FilterSet):
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has_category = django_filters.BooleanFilter(name='category', lookup_expr='isnull', exclude=True)
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Solution 2: Using ``ChoiceFilter``'s null choice
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""""""""""""""""""""""""""""""""""""""""""""""""
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If you're using a ChoiceFilter, you may also filter by null values by enabling
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the ``null_label`` parameter. More details in the ``ChoiceFilter`` reference
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:ref:`docs <choice-filter>`.
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.. code-block:: python
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class ProductFilter(django_filters.FilterSet):
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category = django_filters.ModelChoiceFilter(
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name='category', lookup_expr='isnull',
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null_label='Uncategorized',
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queryset=Category.objects.all(),
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)
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Solution 3: Combining fields w/ ``MultiValueField``
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""""""""""""""""""""""""""""""""""""""""""""""""""
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An alternative approach is to use Django's ``MultiValueField`` to manually add
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in a ``BooleanField`` to handle null values. Proof of concept:
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https://github.com/carltongibson/django-filter/issues/446
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Filtering by an empty string
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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It's not currently possible to filter by an empty string, since empty values are
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interpreted as a skipped filter.
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.. code-block:: http
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GET http://localhost/api/my-model?myfield=
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Solution 1: Magic values
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""""""""""""""""""""""""
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You can override the ``filter()`` method of a filter class to specifically check
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for magic values. This is similar to the ``ChoiceFilter``'s null value handling.
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.. code-block:: http
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GET http://localhost/api/my-model?myfield=EMPTY
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.. code-block:: python
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class MyCharFilter(filters.CharFilter):
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empty_value = 'EMPTY'
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def filter(self, qs, value):
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if value != self.empty_value:
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return super(MyCharFilter, self).filter(qs, value)
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qs = self.get_method(qs)(**{'%s__%s' % (self.name, self.lookup_expr): ""})
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return qs.distinct() if self.distinct else qs
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Solution 2: Empty string filter
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"""""""""""""""""""""""""""""""
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It would also be possible to create an empty value filter that exhibits the same
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behavior as an ``isnull`` filter.
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.. code-block:: http
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GET http://localhost/api/my-model?myfield__isempty=false
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.. code-block:: python
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from django.core.validators import EMPTY_VALUES
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class EmptyStringFilter(filters.BooleanFilter):
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def filter(self, qs, value):
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if value in EMPTY_VALUES:
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return qs
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exclude = self.exclude ^ (value is False)
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method = qs.exclude if exclude else qs.filter
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return method(**{self.name: ""})
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class MyFilterSet(filters.FilterSet):
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myfield__isempty = EmptyStringFilter(name='myfield')
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class Meta:
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model = MyModel
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