Django migrate multiple databases

Question

Environment: Django 2, Python 3.6, PostgreSQL (optional)

I am writing an application with multiple models, some of them might be stored in different databases.

Example:

models.py (might be in different apps)

# this used to allow field 'database' to be processed in 'Meta'
models.options.DEFAULT_NAMES = models.options.DEFAULT_NAMES + ('database', )

class MyModel01(models.Model):
    ...
    # this goes to default database


class MyModel02(models.Model):
    ...

    class Meta:
        database = 'mydb02'


class MyModel03(models.Model):
    ...

    class Meta:
        database = 'mydb03'

routers.py:

class MyRouter(object):
    def db_for_read(self, model, **hints):
        return getattr(model._meta, 'database', None)

    def db_for_write(self, model, **hints):
        return getattr(model._meta, 'database', None)

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        model = hints.get('model')
        if not model:
            return None
        if db != 'default':
            modelname = model_name or model._meta.model_name
            usedb = getattr(model._meta, 'database', None)
            if not usedb:
                return False

            if modelname and usedb:
                return usedb == db
            return False
        else:
            usedb = getattr(model._meta, 'database', 'default')
            return usedb == db
        return None

settings:

DATABASES: {
    'default': {
        # some db settings
    },
    'mydb02': {
        # db settings
    },
    'mydb03': {
        # the SAME db settings as for mydb02
    }
}

The issue is that in settings there might be the same database set for all database aliases.

When I run migrate the migration get applied to just one of the databases, but not always to required one. For example it gets applied to 'default', but not 'mydb03'.

I suppose the mistake is in my router, but cannot find out correct implementation.

It seems the issue is hard to track and to fix. I ended up with having different schemas inside one database (postgresql). — baldr
– baldr, Commented Apr 16, 2018 at 21:20
Do you use any options config? Like this:"'options': -c search_path=schema, public" If yes, try remove the public after comma. — Leonardo Ramos Duarte
– Leonardo Ramos Duarte, Commented Sep 9, 2019 at 20:04
Yes, but this does not solve the issue at all. And you might have not only postgres. — baldr
– baldr, Commented Sep 11, 2019 at 10:57
I have this problem too, with multipls schemas(each schema is a distinct database on django settings), but removing public in options works for me. — Leonardo Ramos Duarte
– Leonardo Ramos Duarte, Commented Sep 11, 2019 at 12:53
In my case they were the same databases and schemas on one of environments. And this is exactly the problem - where migrations table is stored. — baldr
– baldr, Commented Sep 11, 2019 at 14:50

Max Malysh · Accepted Answer · 2018-04-06 14:46:34Z

2

You can specify which database to use:

python3 manage.py migrate # default database
python3 manage.py migrate --database mydb02
python3 manage.py migrate --database mydb03

https://docs.djangoproject.com/en/2.0/topics/db/multi-db/#synchronizing-your-databases

answered Apr 6, 2018 at 14:46

Max Malysh

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2 Comments

baldr Over a year ago

Yes, of course, and this is the only way to run migration for other db aliases. But if 'default' goes first - it might receive the migration, not intended for it. That's the issue.

Leonardo Ramos Duarte Over a year ago

You can config a routerdb to ignore some models in public. It's work for me.

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