django-sql-utils 0.7.0
Improved API for aggregating using Subquery
Requires Python: >=3.8

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Django SQL Utils
================

This package provides utilities for working with Django querysets so that
you can generate the SQL that you want, with an API you enjoy.

Subquery Aggregates
-------------------

The `Count` aggregation in Django::

    Parent.objects.annotate(child_count=Count('child'))

generates SQL like the following::

    SELECT parent.*, Count(child.id) as child_count
    FROM parent
    JOIN child on child.parent_id = parent.id
    GROUP BY parent.id

In many cases, this is not as performant as doing the count in a SUBQUERY
instead of with a JOIN::

    SELECT parent.*,
           (SELECT Count(id)
            FROM child
            WHERE parent_id = parent.id) as child_count
    FROM parent

Django allows us to generate this SQL using The Subquery and OuterRef classes::


    subquery = Subquery(Child.objects.filter(parent_id=OuterRef('id')).order_by()
                        .values('parent').annotate(count=Count('pk'))
                        .values('count'), output_field=IntegerField())
    Parent.objects.annotate(child_count=Coalesce(subquery, 0))

Holy cow! It's not trivial to figure what everything is doing in the above
code and it's not particularly good for maintenance. SubqueryAggregates allow
you to forget all that complexity and generate the subquery count like this::

    Parent.objects.annotate(child_count=SubqueryCount('child'))

Phew! Much easier to read and understand. It's the same API as the original `Count`
just specifying the Subquery version.

Easier API for Exists
---------------------
If you have a Parent/Child relationship (Child has a ForeignKey to Parent), you can annotate a queryset
of Parent objects with a boolean indicating whether or not the parent has children::

    from django.db.models import Exists

    parents = Parent.objects.annotate(
        has_children=Exists(Child.objects.filter(parent=OuterRef('pk'))
    )

That's a bit more boilerplate than should be necessary, so we provide a simpler API for Exists::

    from sql_util.utils import Exists

    parents = Parent.objects.annotate(
        has_children=Exists('child')
    )

The child queryset can be filtered with the keyword argument `filter`. E.g.,::

    parents = Parent.objects.annotate(
        has_child_named_John = Exists('child', filter=Q(name='John'))
    )

The `sql_util` version of `Exists` can also take a queryset as the first parameter and behave just like
the Django `Exists` class, so you are able to use it everywhere without worrying about name confusion.

Installation and Usage
----------------------

Install from PyPI::

    pip install django-sql-utils

Then you can::

    from sql_util.utils import SubqueryCount

And use that as shown above.

In addition to `SubqueryCount`, this package provides

* `SubqueryMin`
* `SubqueryMax`
* `SubquerySum`
* `SubqueryAvg`

If you want to use other aggregates, you can use the
generic `SubqueryAggregate` class. For example, if you want to use Postgres' `ArrayAgg`
to get an array of `Child.name` for each `Parent`::

    from django.contrib.postgres.aggregates import ArrayAgg

    aggregate = SubqueryAggregate('child__name', aggregate=ArrayAgg)
    Parent.objects.annotate(child_names=aggregate)

Or subclass SubqueryAggregate::

    from django.contrib.postgres.aggregates import ArrayAgg

    class SubqueryArrayAgg(SubqueryAggregate)
        aggregate = ArrayAgg
        unordered = True

    Parent.objects.annotate(avg_child_age=SubqueryArrayAgg('child__age'))