Exposition¶
Every feature in whistle-stop detail.
Checks¶
Extra checks added to Django’s check framework to ensure your Django and MySQL configurations are optimal.
$ ./manage.py check
?: (django_mysql.W001) MySQL strict mode is not set for database connection 'default'
...
QuerySet Extensions¶
Django-MySQL comes with a number of extensions to QuerySet that can be
installed in a number of ways - e.g. adding the QuerySetMixin to your
existing QuerySet subclass.
Approximate Counting¶
SELECT COUNT(*) ... can become a slow query, since it requires a scan of
all rows; the approx_count functions solves this by returning the estimated
count that MySQL keeps in metadata. You can call it directly:
Author.objects.approx_count()
Or if you have pre-existing code that calls count() on a QuerySet you
pass it, such as the Django Admin, you can set the QuerySet to do try
approx_count first automatically:
qs = Author.objects.all().count_tries_approx()
# Now calling qs.count() will try approx_count() first
Query Hints¶
Use MySQL’s query hints to optimize the SQL your QuerySets generate:
Author.objects.straight_join().filter(book_set__title__startswith="The ")
# Does SELECT STRAIGHT_JOIN ...
‘Smart’ Iteration¶
Sometimes you need to modify every single instance of a model in a big table, without creating any long running queries that consume large amounts of resources. The ‘smart’ iterators traverse the table by slicing it into primary key ranges which span the table, performing each slice separately, and dynamically adjusting the slice size to keep them fast:
# Some authors to fix
bad_authors = Author.objects.filter(address="Nowhere")
# Before: bad, we can't fit all these in memory
for author in bad_authors.all():
pass
# After: good, takes small dynamically adjusted slices, wraps in atomic()
for author in bad_authors.iter_smart():
author.address = ""
author.save()
author.send_apology_email()
Integration with pt-visual-explain¶
For interactive debugging of queries, this captures the query that the
QuerySet represents, and passes it through EXPLAIN and
pt-visual-explain to get a visual representation of the query plan:
>>> Author.objects.all().pt_visual_explain()
Table scan
rows 1020
+- Table
table myapp_author
MySQL HANDLER API¶
MySQL’s HANDLER commands give simple NoSQL-style read access to rows faster
than normal SQL queries, with the ability to perform index lookups or
page-by-page scans. This extension adds an ORM-based API for handlers:
with Author.objects.handler() as handler:
for author in handler.iter(chunk_size=1000):
author.send_apology_email()
Model Fields¶
Fields that use MySQL-specific features!
JSON Field¶
Implements MySQL 5.7+’s JSON data type for storing arbitrary JSON data:
class APIResponse(Model):
url = models.CharField(max_length=200)
data = JSONField()
>>> APIResponse.objects.create(url='/api/twoots/1/', data={
'id': '123',
'message': 'Loving #django and #mysql',
'coords': [34.4, 56.2]
})
>>> APIResponse.objects.filter(data__coords__0=34.4)
[<APIResponse: /api/twoots/1/>]
Dynamic Columns Field¶
Use MariaDB’s Dynamic Columns for storing arbitrary, nested dictionaries of values:
class ShopItem(Model):
name = models.CharField(max_length=200)
attrs = DynamicField()
>>> ShopItem.objects.create(name='Camembert', attrs={'smelliness': 15})
>>> ShopItem.objects.create(name='Brie', attrs={'smelliness': 5, 'squishiness': 10})
>>> ShopItem.objects.filter(attrs__smelliness_INTEGER__gte=10)
[<ShopItem: Camembert>]
List Fields¶
Two field classes that allow you to store lists of items in a comma-separated string:
class Person(Model):
name = CharField(max_length=32)
post_nominals = ListTextField(
base_field=CharField(max_length=32)
)
>>> Person.objects.filter(post_nominals__contains='PhD')
[<Person: Horatio>, <Person: Severus>]
Set Fields¶
Two field classes that allow you to store sets of items in a comma-separated string:
class Post(Model):
name = CharField(max_length=32)
tags = SetTextField(
base_field=CharField(max_length=10)
)
>>> Post.objects.create(name='First post', tags={'thoughts', 'django'})
>>> Post.objects.filter(tags__contains='django')
[<Post: First post>]
EnumField¶
A field class for using MySQL’s ENUM type, which allows strings that are
restricted to a set of choices to be stored in a space efficient manner:
class BookCover(Model):
color = EnumField(choices=['red', 'green', 'blue'])
Resizable Text/Binary Fields¶
Django’s TextField and
BinaryField fields are fixed at the MySQL level to
use the maximum size class for the BLOB and TEXT data types - these
fields allow you to use the other sizes, and migrate between them:
class BookBlurb(Model):
blurb = SizedTextField(size_class=3)
# Has a maximum length of 16MiB, compared to plain TextField which has
# a limit of 4GB (!)
BIT(1) Boolean Fields¶
Some database systems, such as the Java Hibernate ORM, don’t use MySQL’s
bool data type for storing boolean flags and instead use BIT(1). This
field class allows you to interact with those fields:
class HibernateModel(Model):
some_bool = Bit1BooleanField()
some_nullable_bool = NullBit1BooleanField()
Field Lookups¶
ORM extensions to built-in fields:
>>> Author.objects.filter(name__sounds_like='Robert')
[<Author: Robert>, <Author: Rupert>]
Aggregates¶
MySQL’s powerful GROUP_CONCAT statement is added as an aggregate, allowing
you to bring back the concatenation of values from a group in one query:
>>> author = Author.objects.annotate(
... book_ids=GroupConcat('books__id')
... ).get(name="William Shakespeare")
>>> author.book_ids
"1,2,5,17,29"
Database Functions¶
MySQL-specific database functions for the ORM:
>>> Author.objects.annotate(
... full_name=ConcatWS('first_name', 'last_name', separator=' ')
... ).first().full_name
"Charles Dickens"
Migration Operations¶
MySQL-specific operations for django migrations:
from django.db import migrations
from django_mysql.operations import InstallPlugin
class Migration(migrations.Migration):
dependencies = []
operations = [
InstallPlugin("metadata_lock_info", "metadata_lock_info.so")
]
Cache¶
An efficient backend for Django’s cache framework using MySQL features:
cache.set("my_key", "my_value") # Uses only one query
cache.get_many(["key1", "key2"]) # Only one query to do this too!
cache.set("another_key", some_big_value) # Compressed above 5kb by default
Locks¶
Use MySQL as a locking server for arbitrarily named locks:
with Lock("ExternalAPI", timeout=10.0):
do_some_external_api_stuff()
Status¶
Easy access to global or session status variables:
if global_status.get('Threads_running') > 100:
raise BorkError("Server too busy right now, come back later")
Management Commands¶
dbparams helps you include your database parameters from settings in
commandline tools with dbparams:
$ mysqldump $(python manage.py dbparams) > dump.sql
fix_datetime_columns helps you fix your DateTimeFields that don’t have
microseconds after an upgrade to MySQL 5.6+/MariaDB 5.3+:
$ python manage.py fix_datetime_columns
ALTER TABLE `app1_table1`
MODIFY COLUMN `created_time` datetime(6) DEFAULT NULL;
Utilities¶
Fingerprint queries quickly with the pt-fingerprint wrapper:
>>> pt_fingerprint("SELECT * FROM myapp_author WHERE id = 5")
"select * from myapp_author where id = 5"