django-compoundqueryset 0.1.3

Django-compoundqueryset
------------

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A little Django app allows for compound querysets between different tables.
Especially useful for pagination of said results.



Django versions supported: 1.8, 1.9, 1.10

Python versions supported: 2.7, 3.3, 3.4, 3.5



Installation
------------

You can obtain the source code for ``django-compoundqueryset`` from here:

::

https://github.com/brianwawok/django-compoundqueryset


Using `pip`:

pip install django-compoundqueryset

Motivation
-----------

Sometimes you might want to combine two different tables with similar fields and show them to users. For example:

.. code:: python

from django.db import models



class FirstModel(models.Model):
age = models.IntegerField()
name = models.TextField(max_length=100)
color = models.TextField(max_length=100)


class SecondModel(models.Model):
age = models.IntegerField()
name = models.TextField(max_length=100)
size = models.IntegerField()


And you want to paginate across them

.. code:: python

from django.core.paginator import Paginator
from djcompoundqueryset import CompoundQueryset


qs_1 = FirstModel.objects.order_by('age').all()
qs_2 = SecondModel.objects.order_by('age').all()
combined_queryset = qs_1 | qs_2

p = Paginator(combined_queryset, 10)

You will get the dreaded error

Merging 'QuerySet' classes must involve the same values in each case.

Now with django-compoundqueryset, you can do:

.. code:: python

from django.core.paginator import Paginator
from djcompoundqueryset import CompoundQueryset


qs_1 = FirstModel.objects.order_by('age').all()
qs_2 = SecondModel.objects.order_by('age').all()
combined_queryset = CompoundQueryset(qs_1, qs_2, max_items=100)

p = Paginator(combined_queryset, 10)

You can now iterate over these models in a view, showing the age. Max_items says you will
never use more than that many items, so limit the count a queries. Useful if you have
capped pagination.


Performance
-----------
The goal is to beat the naive implementation (load all records into memory and deal with there), but
we may not beat a RawSQL union all monster. If performance is really important you may need to do that.

If used in a Django Paginator, we will normally do 1 count(*) query per queryset, and then only load the
required records.