django-property-filter 1.3.0
Django-Filter extension to support filtering by Properties
License: MIT
Requires Python: >=3.8

# Django Property Filter

Django-property-filter is an extension to [django-filter](https://pypi.org/projhttps://pypi.org/project/django-filter/ect/django-filter/) and provides functionality to filter querysets by class properties.

It does so by providing sub-classes for Filters and Filtersets to keep existing django-filter functionality.

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For more details and examples check the [documentation](https://django-property-filter.readthedocs.io/en/stable/).

## Requirements

* Python: 3.8, 3.9, 3.10, 3.11, 3.12, 3.13
* Django: 4.2, 5.0, 5.1
* Django-filter: 23.5+

## Limitations

### Performance
Because this app preserved Django Filtersets and filters them agains fields that are not Database fields all filtering happens in memory.
Compared with direct sql optimized queries that might be slower and more memory intensive.

### Limit on returned results
In theory there is no limit for most databases how many results can be returned from a filter query unless the database implements a limit which will impact how many results django-property-filter can return.
Sqlite3 defines different limits depending on the version used.
For further details see the documentation for [limitations](https://django-property-filter.readthedocs.io/en/stable/guide/overview.html#limitations)

## Installation

Install using pip:

```python
pip install django-property-filter
```

Then add 'django_property_filter' to your INSTALLED_APPS.

```python
INSTALLED_APPS = [
    ...
    'django_property_filter',
]
```

## Usage

### Example Model

Our Model

```python
from django.db import models

class BookSeries(models.Model):
    name = models.CharField(max_length=255)

    @property
    def book_count(self):
      return Book.objects.filter(series=self).count()

class Book(models.Model):
    title = models.CharField(max_length=255)
    price = models.DecimalField()
    discount_percentage = models.IntegerField()
    author = models.TextField()
    series = models.ForeignKey(BookSeries)

    @property
    def discounted_price(self):
      return self.price * self.discount_percentage \ 100
```

### Implicit Filter Creation

If we want to filter by discounted price as well as number of books in a series,
which both are properties and not fields in the database, we would do the
following.::

```python
from django_property_filter import PropertyFilterSet, PropertyNumberFilter

class BookFilterSet(PropertyFilterSet):

  class Meta:
      model = Book
      exclude = ['price']
      property_fields = [
        ('discounted_price', PropertyNumberFilter, ['lt', 'exact']),
        ('series.book_count.', PropertyNumberFilter, ['gt', 'exact']),
      ]
```

This will create 4 Filters

    1.) A "less than" and an "exact" filter for the "discounted_price" property
        of the Book Model
    2.) A "greater than" and an "exact" filter for the "book_count" property
        of the related Model "series".

The order in which the property filters are applied are in the same order as
they are defined. In the above filter will be in order of

    1.) discounted_price: lt
    2.) discounted_price: exact
    3.) series.book_count: gt
    4.) series.book_count: exact

This can be usefull to speed up the filtering by property filters if filters
are defined first that are likely to filter a large number of values.
E.g. a boolean field might be filtering an average of half the values, while a
field representing a name will likely filter fewer.

Since PropertyFilterSet is and extension to django-filter's Filterset which
requires either the Meta attribute "fields" or "exclude" to be set we excluded
the "price" field. If we had instead used::
    fields = ['price']

It would also have created an "exact" filter for the book price.

The only difference to using a normal FilterSet from django-filter is the
"property_fields" field.

The "property_fields" is a list of tuples with 3 values.

    1.) The property name. 
        If the property is on a related Model it should be separated by "__",
        and can span multiple levels e.g. fk__fk__fk__property
    2.) The specific Property Filter to use.
        This is necessary since it can't be determined what the return type
        of the property will be in all cases
    3.) The list of lookup expressions.

### Explicit Filter Creation

It is also possible to create Filters explicitely.

```python
from django_property_filter import PropertyNumberFilter, PropertyFilterSet

class BookFilterSet(PropertyFilterSet):
    prop_number_gte = PropertyNumberFilter(field_name='discounted_price', lookup_expr='gte')
    prop_number_lt = PropertyNumberFilter(field_name='discounted_price', lookup_expr='lt')

    class Meta:
        model = NumberClass
        fields = ['prop_number_gte', 'prop_number_lt']
```

This creates a "greater than or equel" and a "less than" filter for the
discounted_price property.

The order in which the property filters are applied are in the same order as
they are defined. In the above filter will be in order of

    1.) prop_number_gte
    2.) prop_number_lt
This can be usefull to speed up the filtering by property filters if filters
are defined first that are likely to filter a large number of values.
E.g. a boolean field might be filtering an average of half the values, while a
field representing a name will likely filter fewer.

## Development

# Run the Django Test Project to see the filters in action

* go to "tests\django_test_proj"
* run "python manage.py makemigrations"
* run "python manage.py migrate"
* run "python manage.py setup_data"
* run "python manage.py runserver"

## Issue Raising ##

Raising bugs, feature requests and questions is possible via the public repository <a href="https://github.com/ericziethen/django-property-filter-issue-reporting" target="_blank">django-property-filter-issue-reporting</a>