How to use a given database to make queries within a method in django?

Question

Here is the case. A method or function has a couple of database queries. There are two databases, such as DATABASE_A and DATABASE_B.

def method():
    # database queries here
    orders = Order.objects.filter(...)
    products = Product.objects.filter(...)

How to achieve the follows easily with decorator or any other ways? Can anyone give an example? model.objects.using(some database) is a known way, however there are a lot of queries in the method. I don't want to touch them, so this is not an option.

# Here call method() using DATABASE_A
# here call method() using DATABASE_B

marcanuy · Accepted Answer · 2018-06-08 16:48:51Z

1

As specified in Multiple databases docs:

You can select the database for a QuerySet at any point in the QuerySet “chain.” Just call using() on the QuerySet to get another QuerySet that uses the specified database.

>>> # This will run on the 'default' database.
>>> Author.objects.all()

>>> # So will this.
>>> Author.objects.using('default').all()

>>> # This will run on the 'other' database.
>>> Author.objects.using('other').all()

answered Jun 8, 2018 at 16:48

marcanuy

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1 Comment

caot Over a year ago

using(somedb) is a known way, however there are a lot of queries in the method. I don't want to touch them. So the way you given is not an option.

caot · Accepted Answer · 2018-06-08 23:09:49Z

Django, Databases, and Decorators has a solution using django DATABASE_ROUTERS.

Here you can find an example from docs.djangoproject.com. The follows is the summary from 'Django, Databases, and Decorators':

1. Writing a custom decorator

Here name the decorator as decorators.py:

from functools import wraps

try:
    from threading import local
except ImportError:
    from _threading_local import local

threadlocal = local()


class use_db_for_reads(object):

    def __init__(self, database_name):
        self.database_name = database_name

    def __enter__(self):
        setattr(threadlocal, 'DB_FOR_READ_ONLY', self.database_name)

    def __exit__(self, exc_type, exc_value, traceback):
        setattr(threadlocal, 'DB_FOR_READ_ONLY', None)

    def __call__(self, test_func):
        @wraps(test_func)
        def inner(*args, **kwargs):
            return test_func(*args, **kwargs)
        return inner


def get_thread_local(attr, default=None):
    return getattr(threadlocal, attr, default)


class AnalyticsRouter(object):

    def db_for_read(self, model, **hints):
        return get_thread_local('DB_FOR_READ_ONLY', 'default')

    def db_for_write(self, model, **hints):
        return 'default'

    def allow_relation(self, obj1, obj2, **hints):
        return True

2. update settings

Here is an example of DATABASES in settings.py:

DATABASES = {
    'default': {
        ...
    },
    'read-only': {
        ...
    }
}

DATABASE_ROUTERS = ['decorators.AnalyticsRouter']

3. Here is how to use it

from decorators import use_db_for_reads

# Here call method() using DATABASE_A
with use_db_for_reads(DATABASE_A):
    method()

# here call method() using DATABASE_B
with use_db_for_reads(DATABASE_B):
    method()

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