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https://github.com/miki725/django-sub-query

Django app which uses SQL sub-queries to solve some ORM limitations
https://github.com/miki725/django-sub-query

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Django app which uses SQL sub-queries to solve some ORM limitations

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README 

        
===============

Django SubQuery

===============



.. image:: https://badge.fury.io/py/django-sub-query.png

    :target: http://badge.fury.io/py/django-sub-query



.. image:: https://travis-ci.org/miki725/django-sub-query.png?branch=master

    :target: https://travis-ci.org/miki725/django-sub-query



Django app which uses SQL sub-queries to solve some ORM limitations



* Free software: MIT license

* GitHub: https://github.com/miki725/django-sub-query



Why?

----



Django ORM is pretty awesome however it has some limitations.

One of such limitations is the ability to use sub-queries.

Actually thats not 100% true since Django ORM can use nested queries

however those subqueries are for either aggregates or are manually

provided by the user via ``QuerySet.extra()``. Such types of subqueries

however sometimes are not good enough. One such scenario is when

you need to sort by one column however and at the same time you need to use

distinct on a different column. The usual solution is to use subquery

where inner query will use distinct and outer query will do the sorting::



    SELECT *

    FROM (

      SELECT DISTINCT ON ("table"."foo") 

      FROM "table"

    ) "table"

    ORDER BY "table"."bar" ASC;



The above however is not supported by vanilla Django ORM.

You can of course use raw SQL queries however that is not desired since

then you loose all of the ORM power (e.g. pagination, etc).



One hack to still use ORM and yet use subquery is use Django ORM

to construct inner query and then manually add outer query::



    query = Model.objects.filter(...).distinct(...).query

    sql, params = query.sql_with_params()

    Model.objects.raw(

        'SELECT * FROM ({}) "table" ORDER BY "table"."foo"'.format(sql),

        params

    )



This approach however is also not desired since due to:



* not able to use ``select_related()`` when related table

  has similar column name(s)

* still difficult to do pagination

* more logic required to be able to do ``.count()``



Solution

--------



As you saw above, currently it is difficult to use sub-queries in Django

however conceptually the solution is pretty simply since we should be able

to simply wrap SQL generated by Django ORM within an outer query.

This is exactly what this library does. It hooks up directly into Django's

``as_sql()`` ORM method which is responsible for generating SQL and adds

outer query when necessary conditions are met.



Installing

----------



You can install ``django-sub-query`` using pip::



    $ pip install django-sub-query



Using

-----



Since this library changes a few bits in inner-workings in Django ORM,

there is no simple way to use this library other then completely change

database engine::



    DATABASES = {

        'default': {

            'ENGINE': 'sub_query.db.backends.postgis',

            ...

        },

    }



Currently only ``postgis`` backend is supported however please feel free to

open an issue to add support for other backends (or contribute implementation!).



Once the database engine is changed, then you have to make sure the

queryset you are using is a subclass of ``SubQueryGeoQuerySet``::



    class ExampleModel(models.Model):

        ...

        objects = SubQueryGeoQuerySet.as_manager()



Testing

-------



To run the tests you need to install testing requirements first::



    $ make install



Then to run tests, you can use ``manage.py`` or simply use Makefile command::



    $ python manage.py test

    # or

    $ make test