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Connect a second database

Goal: query a second (or third+) database from a controller, alongside the app’s primary ctx.db connection — for example, a read replica, a legacy database, or per-tenant databases.

Loco ships a ready-made initializer for this: MultiDbInitializer, a named map of connections. It lives under loco_rs::initializers::multi_db and is gated behind the with-db feature. Each config entry accepts the same keys as the primary database: block — see Configuration § database for the full list (uri, enable_logging, min_connections, max_connections, connect_timeout, idle_timeout, acquire_timeout, auto_migrate, dangerously_truncate, dangerously_recreate, run_on_start).

Configure each named database

Add one or more entries under a multi_db map, nested under the top-level initializers key in your environment config. A single extra connection is just a one-entry map:

initializers:
  multi_db:
    secondary_db:
      uri: postgres://loco:loco@localhost:5432/loco_app
      enable_logging: false
      connect_timeout: 500
      idle_timeout: 500
      min_connections: 1
      max_connections: 1
      auto_migrate: false
      dangerously_truncate: false
      dangerously_recreate: false

Add more entries to open more connections:

initializers:
  multi_db:
    secondary_db:
      uri: postgres://loco:loco@localhost:5432/loco_app
      enable_logging: false
      connect_timeout: 500
      idle_timeout: 500
      min_connections: 1
      max_connections: 1
      auto_migrate: false
      dangerously_truncate: false
      dangerously_recreate: false
    third_db:
      uri: postgres://loco:loco@localhost:5432/loco_app_reporting
      enable_logging: false
      connect_timeout: 500
      idle_timeout: 500
      min_connections: 1
      max_connections: 1
      auto_migrate: false
      dangerously_truncate: false
      dangerously_recreate: false

Register the initializer

use loco_rs::app::{AppContext, Initializer};


async fn initializers(_ctx: &AppContext) -> Result<Vec<Box<dyn Initializer>>> {
    let initializers: Vec<Box<dyn Initializer>> = vec![
        Box::new(loco_rs::initializers::multi_db::MultiDbInitializer),
    ];


    Ok(initializers)
}

Look connections up by name

MultiDbInitializer layers a loco_rs::db::MultiDb (a thin HashMap<String, DatabaseConnection> wrapper) as an axum Extension:

use sea_orm::EntityTrait;
use axum::{response::IntoResponse, Extension};
use loco_rs::db::MultiDb;


pub async fn list(
    State(ctx): State<AppContext>,
    Extension(multi_db): Extension<MultiDb>,
) -> Result<impl IntoResponse> {
    let third_db = multi_db.get("third_db")?;
    let res = Entity::find().all(third_db).await;
    format::json(res)
}

multi_db.get(name) returns an error if that key isn’t configured — no silent None/panic.

Result

ctx.db remains your app’s primary connection (used for auto-migration, boot-time checks, etc.); the extra connection(s) arrive purely through the MultiDb axum Extension and only in handlers that ask for them.

Migrating from extra_db

ExtraDbInitializer has been removed in favor of MultiDbInitializer. To migrate:

  • Config: former initializers.extra_db: { ... } becomes initializers.multi_db: { <name>: { ... } } — pick a name for your connection and nest the same keys under it.
  • Registration: swap Box::new(loco_rs::initializers::extra_db::ExtraDbInitializer) for Box::new(loco_rs::initializers::multi_db::MultiDbInitializer).
  • Handlers: change Extension(db): Extension<DatabaseConnection> to Extension(multi_db): Extension<MultiDb>, then look up the connection with let db = multi_db.get("<name>")?;.

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