Multiple Databases¶

SQLAlchemy has the capability to establish connections with multiple databases simultaneously, referring to these connections as "binds."

EllarSQL simplifies the management of binds by associating each engine with a short string identifier, __database__. Subsequently, each model and table is linked to a __database__, and during a query, the session selects the appropriate engine based on the __database__ of the entity being queried. In the absence of a specified __database__, the default engine is employed.

Configuring Multiple Databases¶

In EllarSQL, database configuration begins with the setup of the default database, followed by additional databases, as exemplified in the EllarSQLModule configurations:

from ellar_sql import EllarSQLModule

EllarSQLModule.setup(
    databases={
        "default": "postgresql:///main",
        "meta": "sqlite:////path/to/meta.db",
        "auth": {
            "url": "mysql://localhost/users",
            "pool_recycle": 3600,
        },
    },
    migration_options={'directory': 'migrations'}
)

Defining Models and Tables with Different Databases¶

EllarSQL creates Metadata and an Engine for each configured database. Models and tables associated with a specific __database__ key are registered with the corresponding Metadata. During a session query, the session employs the related Engine.

To designate the database for a model, set the __database__ class attribute. Not specifying a __database__ key is equivalent to setting it to default:

In Models¶

from ellar_sql import model

class User(model.Model):
    __database__ = "auth"
    id = model.Column(model.Integer, primary_key=True)

Models inheriting from an already existing model will share the same database key unless they are overriden.

Info

Its importance to not that model.Model has __database__ value equals default

In Tables¶

To specify the database for a table, utilize the __database__ keyword argument:

from ellar_sql import model

user_table = model.Table(
    "user",
    model.Column("id", model.Integer, primary_key=True),
    __database__="auth",
)

Info

Ultimately, the session references the database key associated with the metadata or table, an association established during creation. Consequently, changing the database key after creating a model or table has no effect.

Creating and Dropping Tables¶

The create_all() and drop_all() methods operating are all part of the EllarSQLService. It also requires the database argument to target a specific database.

# Create tables for all binds
from ellar.app import current_injector
from ellar_sql import EllarSQLService

db_service = current_injector.get(EllarSQLService)

# Create tables for all configured databases
db_service.create_all()

# Create tables for the 'default' and "auth" databases
db_service.create_all('default', "auth")

# Create tables for the "meta" database
db_service.create_all("meta")

# Drop tables for the 'default' database
db_service.drop_all('default')