91 lines
3.4 KiB
Plaintext
91 lines
3.4 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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