django-generic-aggregation

annotate() and aggregate() for generically-related data. also a handy function for filtering GFK-model querysets.

Note

Use django’s GenericRelation where possible, as this can make the queries generated more efficient by using a JOIN rather than a subquery.

installation

# install from pypi
pip install django-generic-aggregation

# or install via git
pip install -e git+git://github.com/coleifer/django-generic-aggregation.git#egg=generic_aggregation

examples

The examples below assume the following simple models:

class Rating(models.Model):
    rating = models.IntegerField()
    object_id = models.IntegerField()
    content_type = models.ForeignKey(ContentType)
    content_object = GenericForeignKey(ct_field='content_type', fk_field='object_id')

class Food(models.Model):
    name = models.CharField(max_length=50)
    ratings = generic.GenericRelation(Rating) # reverse generic relation

You want to figure out which items are highest rated (generic_annotate())

from django.db.models import Avg

food_qs = Food.objects.filter(name__startswith='a')
generic_annotate(food_qs, Rating, Avg('ratings__rating'))

# you can mix and match queryset / model
generic_annotate(food_qs, Rating.objects.all(), Avg('ratings__rating'))

You want the average rating for all foods that start with ‘a’ (generic_aggregate())

food_qs = Food.objects.filter(name__startswith='a')
generic_aggregate(food_qs, Rating, Avg('ratings__rating'))

You want to only display ratings for foods that start with ‘a’ (generic_filter())

food_qs = Food.objects.filter(name__startswith=’a’) generic_filter(Rating.objects.all(), food_qs)

important detail

As you may have noted in the above examples (at least those using annotate and aggregate), the aggregate we pass in is prefixed with ratings__. The double-underscore prefix refers to the ratings attribute of the Food model, which is a django.contrib.contenttypes.generic.GenericRelation instance. We are querying across that relation to the field on the Ratings model that we are interested in. When possible, use a GenericRelation and construct your queries in this manner.

If you do not have a GenericRelation on the model being queried, it will use a “fallback” method that will return the correct results, though queried in a slightly different manner (a subquery will be used as opposed to a left outer join).

If for some reason the Generic Foreign Key’s “object_id” field is of a different type than the Primary Key of the related model – which is probably the case if you’re using django.contrib.comments, as it uses a TextField – a CAST expression is required by some RDBMS’. Django will not put it there for you, so again, the code will use the “fallback” methods in this case, which add the necessary CAST.

View the code for the nitty-gritty details.

api

generic_aggregation.generic_annotate(qs_model, generic_qs_model, aggregator[, gfk_field=None[, alias='score']])

Find blog entries with the most comments:

qs = generic_annotate(Entry.objects.public(), Comment.objects.public(), Count('comments__id'))
for entry in qs:
    print entry.title, entry.score

Find the highest rated foods:

generic_annotate(Food, Rating, Avg('ratings__rating'), alias='avg')
for food in qs:
    print food.name, '- average rating:', food.avg

Note

In both of the above examples it is assumed that a GenericRelation exists on Entry to Comment (named “comments”) and also on Food to Rating (named “ratings”). If a GenericRelation does not exist, the query will still return correct results but the code path will be different as it will use the fallback method.

Warning

If the underlying column type differs between the qs_model’s primary key and the generic_qs_model’s foreign key column, it will use the fallback method, which can correctly CASTself.

Parameters:

• qs_model – A model or a queryset of objects you want to perform annotation on, e.g. blog entries
• generic_qs_model – A model or queryset containing a GFK, e.g. comments
• aggregator – an aggregation, from django.db.models, e.g. Count(‘id’) or Avg(‘rating’)
• gfk_field – explicitly specify the field w/the gfk
• alias – attribute name to use for annotation

Return type:

a queryset containing annotate rows

generic_aggregation.generic_aggregate(qs_model, generic_qs_model, aggregator[, gfk_field=None])

Find total number of comments on blog entries:

generic_aggregate(Entry.objects.public(), Comment.objects.public(), Count('comments__id'))

Find the average rating for foods starting with ‘a’:

a_foods = Food.objects.filter(name__startswith='a')
generic_aggregate(a_foods, Rating, Avg('ratings__rating'))

Note

In both of the above examples it is assumed that a GenericRelation exists on Entry to Comment (named “comments”) and also on Food to Rating (named “ratings”). If a GenericRelation does not exist, the query will still return correct results but the code path will be different as it will use the fallback method.

Warning

If the underlying column type differs between the qs_model’s primary key and the generic_qs_model’s foreign key column, it will use the fallback method, which can correctly CASTself.

Parameters:

• qs_model – A model or a queryset of objects you want to perform annotation on, e.g. blog entries
• generic_qs_model – A model or queryset containing a GFK, e.g. comments
• aggregator – an aggregation, from django.db.models, e.g. Count(‘id’) or Avg(‘rating’)
• gfk_field – explicitly specify the field w/the gfk

Return type:

a scalar value indicating the result of the aggregation

generic_aggregation.generic_filter(generic_qs_model, filter_qs_model[, gfk_field=None])

Only show me ratings made on foods that start with “a”:

a_foods = Food.objects.filter(name__startswith=’a’) generic_filter(Rating.objects.all(), a_foods)

Only show me comments from entries that are marked as public:

generic_filter(Comment.objects.public(), Entry.objects.public())

Parameters:

• generic_qs_model – A model or queryset containing a GFK, e.g. comments
• qs_model – A model or a queryset of objects you want to restrict the generic_qs to
• gfk_field – explicitly specify the field w/the gfk

Return type:

a filtered queryset

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