id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
14121	Provide object's primary key as a hint for database router method db_for_read() when filtering objects by primary key	sandy@…	nobody	"'''Problem:'''

Setting up a custom database router with db_for_read() method to allow for sharding with MySQL. Need to be able to control what database an object is read from on a per-object basis.
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'''Example Case:'''

I want to retrieve User objects from the database cluster as follows:

    * If the username starts with a number, read from slave_db_1

    * If the username starts with a letter, read from slave_db_2
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Currently, the only way to achieve this behavior is to specify ""using"" on each condition, and per each call to the database.
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'''Proposed Solution:'''
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When filtering objects by primary key, pass the primary key to db_for_read() as a hint. 
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'''Example Code:'''


{{{
#View Code:
""""""Assume username is pk for User model""""""
users = User.objects.get(username=""testuser"")

#DB Router Code
class UserReadRouter(object):
    def db_for_write(self, model, **hints):
        return 'master'

    def db_for_read(self, model, **hints):
        """"""Assume pk of User object is a string""""""
        if model.__name__ == ""User"" and 'object_pk' in hints:
	    if hints['object_pk'][0].isalpha():
                return 'slave_db_2'
            else:
                return 'slave_db_1'
}}}
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The end result is that ""testuser"" is retrieved from slave_db_1."	New feature	closed	Database layer (models, ORM)	1.2	Normal	wontfix	router, hints	noah@…	Design decision needed	0	0	0	0	0	0