Package: django-batch-select
The idea of Django Batch Select is to provide an equivalent to
Django’sselect_relatedfunctionality.  As of such it’s another handy
tool for avoiding the “n+1 query problem”.select_relatedis handy for minimizing the number of queries that need
to be made in certain situations.  However it is only usual for
pre-selectingForeignKeyrelations.batch_select is handy for pre-selectingManyToManyFieldrelations and
reverseForeignKeyrelations.It works by performing a single extra SQL query after aQuerySethas
been evaluated to stitch in the the extra fields asked for.  This requires
the addition of a customManager, which in turn returns a customQuerySetwith extra methods attached.Example UsageAssuming we have models defined as the following:from batch_select.models import BatchManager

class Tag(models.Model):
    name = models.CharField(max_length=32)

class Section(models.Model):
    name = models.CharField(max_length=32)

    objects = BatchManager()

class Entry(models.Model):
    title = models.CharField(max_length=255)
    section = models.ForeignKey(Section, blank=True, null=True)
    tags = models.ManyToManyField(Tag)

    objects = BatchManager()I’ll also define a helper function to show the SQL queries generated:from django import db

def show_queries():
    for query in db.connection.queries:
        print query["sql"]
    db.reset_queries()Here are a few example (with generated sql queries):>>> Entry.objects.batch_select('tags').all()
[]
>>> show_queries() # no results, so no 2nd query
SELECT "batch_select_entry"."id", "batch_select_entry"."title", "batch_select_entry"."section_id" FROM "batch_select_entry"
>>> Entry.objects.create()
>>> Entry.objects.create()
>>> tag1 = Tag.objects.create(name='tag1')
>>> tag2 = Tag.objects.create(name='tag2')
>>> db.reset_queries()
>>> entries = Entry.objects.batch_select('tags').all()
>>> entry = entries[0]
>>> print entry.tags_all
[]
>>> show_queries()
SELECT "batch_select_entry"."id", "batch_select_entry"."title", "batch_select_entry"."section_id" FROM "batch_select_entry" LIMIT 1
SELECT (`batch_select_entry_tags`.`entry_id`) AS "entry_id", "batch_select_tag"."id", "batch_select_tag"."name" FROM "batch_select_tag" INNER JOIN "batch_select_entry_tags" ON ("batch_select_tag"."id" = "batch_select_entry_tags"."tag_id") WHERE "batch_select_entry_tags".entry_id IN (1)
>>> entry.tags.add(tag1)
>>> db.reset_queries()
>>> entries = Entry.objects.batch_select('tags').all()
>>> entry = entries[0]
>>> print entry.tags_all
[<Tag: Tag object>]
>>> show_queries()
SELECT "batch_select_entry"."id", "batch_select_entry"."title", "batch_select_entry"."section_id" FROM "batch_select_entry" LIMIT 1
SELECT (`batch_select_entry_tags`.`entry_id`) AS "entry_id", "batch_select_tag"."id", "batch_select_tag"."name" FROM "batch_select_tag" INNER JOIN "batch_select_entry_tags" ON ("batch_select_tag"."id" = "batch_select_entry_tags"."tag_id") WHERE "batch_select_entry_tags".entry_id IN (1)
>>> entries = Entry.objects.batch_select('tags').all()
>>> for entry in entries:
....     print entry.tags_all
....
[<Tag: Tag object>]
[]
>>> show_queries()
SELECT "batch_select_entry"."id", "batch_select_entry"."title", "batch_select_entry"."section_id" FROM "batch_select_entry"
SELECT (`batch_select_entry_tags`.`entry_id`) AS "entry_id", "batch_select_tag"."id", "batch_select_tag"."name" FROM "batch_select_tag" INNER JOIN "batch_select_entry_tags" ON ("batch_select_tag"."id" = "batch_select_entry_tags"."tag_id") WHERE "batch_select_entry_tags".entry_id IN (1, 2)Re-running that same last for loop without using batch_select
generate three queries instead of two (n+1 queries):>>> entries = Entry.objects.all()
>>> for entry in entries:
....     print entry.tags.all()
....
[<Tag: Tag object>]
[]

>>> show_queries()
SELECT "batch_select_entry"."id", "batch_select_entry"."title", "batch_select_entry"."section_id" FROM "batch_select_entry"
SELECT "batch_select_tag"."id", "batch_select_tag"."name" FROM "batch_select_tag" INNER JOIN "batch_select_entry_tags" ON ("batch_select_tag"."id" = "batch_select_entry_tags"."tag_id") WHERE "batch_select_entry_tags"."entry_id" = 1
SELECT "batch_select_tag"."id", "batch_select_tag"."name" FROM "batch_select_tag" INNER JOIN "batch_select_entry_tags" ON ("batch_select_tag"."id" = "batch_select_entry_tags"."tag_id") WHERE "batch_select_entry_tags"."entry_id" = 2This also works with reverse foreign keys. So for example we can get
this entries that belong to each section:>>> section1 = Section.objects.create(name='section1')
>>> section2 = Section.objects.create(name='section2')
>>> Entry.objects.create(section=section1)
>>> Entry.objects.create(section=section1)
>>> Entry.objects.create(section=section2)
>>> db.reset_queries()
>>> Section.objects.batch_select('entry_set')
[<Section: Section object>, <Section: Section object>]
>>> show_queries()
SELECT "batch_select_section"."id", "batch_select_section"."name" FROM "batch_select_section" LIMIT 21
SELECT ("batch_select_entry"."section_id") AS "__section_id", "batch_select_entry"."id", "batch_select_entry"."title", "batch_select_entry"."section_id", "batch_select_entry"."location_id" FROM "batch_select_entry" WHERE "batch_select_entry"."section_id" IN (1, 2)Each section object in that query will have an entry_set_all field
containing the relevant entries.You need to pass batch_select the “related name” of the foreign key,
in this case “entry_set”. NB by default the related name for a foreign
key does not actually include the _set suffix, so you can use just “entry”
in this case. I have made sure that the _set suffix version also works to
try and keep the API simpler.More Advanced UsageBy default the batch fields are inserted into fields named<name>_all,
on each object.  So:Entry.objects.batch_select('tags').all()results in the Entry instances having fields called'tags_all'containing the Tag objects associated with that Entry.If you want to give the field a different name just use a keyword
argument - in the same way as using theAggregationAPI:Entry.objects.batch_select(selected_tags='tags').all()Would means the Tag objects would be assigned to fields called'selected_tags'.If you want to perform filtering of the related objects you will need to
use a Batch object. By doing this you can pass extra keyword arguments
in the same way as when using the filter method of a QuerySet:from batch_select.models import Batch

Entry.objects.batch_select(tags_containing_blue=Batch('tags', name__contains='blue'))Would return Entry objects with fields called ‘tags_containing_name’ with
only those Tags whose name contains ‘blue’.In addition to filtering using keyword arguments, you can also call the
following methods on a Batch object, with their effects being passed on
to the underlyingQuerySetobject:filterexcludeannotateorder_byreverseselect_relatedextradeferonlybatch_select(Note that distinct(), values() etc are not included as they would have
side-effects on how the extra query is associated with the original query)
So for example to achieve the same effect as the filter above you could
do the following:from batch_select.models import Batch

Entry.objects.batch_select(tags_containing_blue=Batch('tags').filter(name__contains='blue'))Whereas the following would exclude tags containing “blue” and order by name:from batch_select.models import Batch

batch = Batch('tags').exclude(name__contains='blue').order_by('name')
Entry.objects.batch_select(tags_not_containing_blue=batch)CompatibilityDjango batch select should work with Django 1.1-1.3 at least.TODOs and BUGSSee:http://github.com/lilspikey/django-batch-select/issues