Migrations, Transactions & Async Considerations

Versioned Migrations

diesel migration generate create_users
# creates migrations/<timestamp>_create_users/{up.sql,down.sql}

diesel migration run      # applies pending migrations, regenerates schema.rs
diesel migration revert   # rolls back the most recent migration

Each migration is an ordered up.sql/down.sql pair. diesel migration run applies pending changes and refreshes schema.rs so the type-checked query builder stays aligned with the real database.

Transactions

conn.transaction::<_, diesel::result::Error, _>(|conn| {
    diesel::insert_into(users::table).values(&new_user).execute(conn)?;
    diesel::update(accounts::table.find(account_id))
        .set(accounts::balance.eq(new_balance))
        .execute(conn)?;
    Ok(())
})?;

The transaction closure commits automatically on Ok and rolls back on Err — a clean way to group multiple queries into one atomic operation without manual BEGIN/COMMIT/ROLLBACK.

Diesel Is Synchronous — Handling Async Web Servers

Classic Diesel connections are blocking; calling them directly inside an async Axum/Actix handler stalls the executor thread. The common fix is running Diesel calls via tokio::task::spawn_blocking on a dedicated thread pool, or adopting the diesel-async crate for a native async connection — a real architectural decision to make early in a Tokio-based project.

Diesel vs. sqlx

Both give compile-time query safety, via different mechanisms: Diesel validates a typed query-builder DSL against a generated schema, while sqlx's checked macros validate literal SQL strings against live database metadata at build time. Diesel trades some flexibility for its DSL's structure; sqlx keeps queries as plain SQL text but still catches mismatches before runtime.