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13. How to add multiple databases to the django application ?

The configuration of database related stuff is mostly done in settings.py file. So to add multiple database to our django project we need add them in DATABASES dictionary.

DATABASE_ROUTERS = ['path.to.DemoRouter']
DATABASE_APPS_MAPPING = {'user_data': 'users_db',
                        'customer_data':'customers_db'}

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
    },
    'users_db': {
        'NAME': 'user_data',
        'ENGINE': 'django.db.backends.postgresql',
        'USER': 'postgres_user',
        'PASSWORD': 'password'
    },
    'customers_db': {
        'NAME': 'customer_data',
        'ENGINE': 'django.db.backends.mysql',
        'USER': 'mysql_cust',
        'PASSWORD': 'root'
    }
}

With multiple databases it will be good to talk about Database Router. The default routing scheme ensures that if a database isn’t specified, all queries fall back to the default database. Database Router defaults to [].

class DemoRouter:
    """
    A router to control all database operations on models in the
    user application.
    """
    def db_for_read(self, model, **hints):
        """
        Attempts to read user models go to users_db.
        """
        if model._meta.app_label == 'user_data':
            return 'users_db'
        return None

    def db_for_write(self, model, **hints):
        """
        Attempts to write user models go to users_db.
        """
        if model._meta.app_label == 'user_data':
            return 'users_db'
        return None

    def allow_relation(self, obj1, obj2, **hints):
        """
        Allow relations if a model in the user app is involved.
        """
        if obj1._meta.app_label == 'user_data' or \
           obj2._meta.app_label == 'user_data':
           return True
        return None

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """
        Make sure the auth app only appears in the 'users_db'
        database.
        """
        if app_label == 'user_data':
            return db == 'users_db'
        return None

Respective models would be modified as

class User(models.Model):
    username = models.Charfield(ax_length=100)
    . . .
        class Meta:
        app_label = 'user_data'

class Customer(models.Model):
    name = models.TextField(max_length=100)
    . . .
        class Meta:
        app_label = 'customer_data'

Few helpful commands while working with multiple databases.

$ ./manage.py migrate --database=users_db
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