Advanced Django ORM techniques

• 

  Indra Singh

  8 months ago
• 

  Bangyun Hu , Supervisor at VeryCD

  12 months ago
• 

  Yuriy Yurchenko , Project manager (CUSTIS) at CUSTIS

  4 years ago
• 

  Josh Daeseon Kim , Software Engineer at Samsung Electronics at Samsung Electronics

  4 years ago
• 

  zhao hailong , Server developer at 数字天空

  4 years ago

(background: montage of Limmud, rosemanblog, Capital, Classic, Heart, GlassesDirect)

Some of same ideas in Guido's Appstats talk this morning

It's a model, in a field, geddit?

For more, see Marty Alchin, Pro Django (Apress)

descriptors used especially in related objects - see later

Very useful for introspection and working out what's going on

explain identity: multiple instances relating to same model row aren't the same object, changes made to one don't reflect the other; even saving one with new values won't be reflected in others.

Update, Aggregates, Q, F

Find repeated queries with my branch of the django-debug-toolbar, or SimonW's original query debug middleware

Actually in 1.2 there's an extra _state object in __dict__, which is used for the multiple DB support (which I'm not covering here).

Lack of model identity means that accessing the related item on one instance does not cause cache to be created on other instances that might reference the same db row

Note: backwards cache does work on OneToOne as of 1.2

+----+--------------------+-----------+-----------------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-----------+-----------------+---------------+---------+---------+------+------+-------------+
| 1 | PRIMARY | sandy_bar | ALL | NULL | NULL | NULL | NULL | 100 | Using where |
| 2 | DEPENDENT SUBQUERY | U0 | unique_subquery | PRIMARY | PRIMARY | 4 | func | 1 | Using where |
+----+-----------+----+--------------------+-----------+-----------------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-----------+-----------------+---------------+---------+---------+------+------+-------------+
| 1 | PRIMARY | sandy_bar | ALL | NULL | NULL | NULL | NULL | 100 | Using where |
| 2 | DEPENDENT SUBQUERY | U0 | unique_subquery | PRIMARY | PRIMARY | 4 | func | 1 | Using where |
+----+--------------------+-----------+-----------------+---------------+---------+---------+------+------+-------------+

--------+-----------+-----------------+---------------+---------+---------+------+------+-------------+