feat: support attached databases

- This PR plumbs the feature through (TS -> commands -> wrapper)
- Introducing a new builder pattern to chain the attach() fn
  - The attach() fn can be chained with any query fn
  - This prevents introducing optional attached db params everywhere
- Unfortunately we never set up eslint in this repo so this PR adds that
  - This has resulted in a lot of changes to the TS files
  - However this is a good time to do it because the builder pattern
     meant significant changes to the TS anyway
- Also doubled back to add tests for the new attached.rs API which was
   added in the last PR

Author: pmorris-dev

README.md

@@ -289,6 +289,67 @@ To rollback instead of committing:
 await tx.rollback()
 ```
 
+### Cross-Database Queries
+
+Attach other SQLite databases to run queries across multiple database files.
+Each attached database gets a schema name that acts as a namespace for its
+tables.
+
+**Builder Pattern:** All query methods (`execute`, `executeTransaction`,
+`fetchAll`, `fetchOne`) return builders that support `.attach()` for
+cross-database operations.
+
+```typescript
+// Join data from multiple databases
+const results = await db.fetchAll(
+   'SELECT u.name, o.total FROM users u JOIN orders.orders o ON u.id = o.user_id',
+   []
+).attach([
+   {
+      databasePath: 'orders.db',
+      schemaName: 'orders',
+      mode: 'readOnly'
+   }
+])
+
+// Update main database using data from attached database
+await db.execute(
+   'UPDATE todos SET status = $1 WHERE id IN (SELECT todo_id FROM archive.completed)',
+   ['archived']
+).attach([
+   {
+      databasePath: 'archive.db',
+      schemaName: 'archive',
+      mode: 'readOnly'
+   }
+])
+
+// Atomic writes across multiple databases
+await db.executeTransaction([
+   ['INSERT INTO main.orders (user_id, total) VALUES ($1, $2)', [userId, total]],
+   ['UPDATE stats.order_count SET count = count + 1', []]
+]).attach([
+   {
+      databasePath: 'stats.db',
+      schemaName: 'stats',
+      mode: 'readWrite'
+   }
+])
+```
+
+**Attached Database Modes:**
+
+   * `readOnly` - Read-only access (can be used with read or write operations)
+   * `readWrite` - Read-write access (requires write operation, holds write
+     lock)
+
+**Important:**
+
+   * Attached database(s) automatically detached after query completion
+   * Read-write attachments acquire write locks on all involved databases
+   * Attachments are connection-scoped and don't persist across queries
+   * Main database is always accessible without a schema prefix
+
 ### Error Handling
 
 ```typescript
@@ -315,9 +376,9 @@ Common error codes:
 ### Closing and Removing
 
 ```typescript
-await db.close()           // Close this connection
-await Database.closeAll()  // Close all connections
-await db.remove()          // Close and DELETE database file(s) - irreversible!
+await db.close()            // Close this connection
+await Database.close_all()  // Close all connections
+await db.remove()           // Close and DELETE database file(s) - irreversible!
 ```
 
 ## API Reference
@@ -328,7 +389,7 @@ await db.remove()          // Close and DELETE database file(s) - irreversible!
 | ------ | ----------- |
 | `Database.load(path, config?)` | Connect and return Database instance (or existing) |
 | `Database.get(path)` | Get instance without connecting (lazy init) |
-| `Database.closeAll()` | Close all database connections |
+| `Database.close_all()` | Close all database connections |
 
 ### Instance Methods
 
@@ -342,6 +403,16 @@ await db.remove()          // Close and DELETE database file(s) - irreversible!
 | `close()` | Close connection, returns `true` if was loaded |
 | `remove()` | Close and delete database file(s), returns `true` if was loaded |
 
+### Builder Methods
+
+All query methods (`execute`, `executeTransaction`, `fetchAll`, `fetchOne`)
+return builders that are directly awaitable and support method chaining:
+
+| Method | Description |
+| ------ | ----------- |
+| `attach(specs)` | Attach databases for cross-database queries, returns `this` |
+| `await builder` | Execute the query (builders implement `PromiseLike`) |
+
 ### InterruptibleTransaction Methods
 
 | Method | Description |
@@ -364,6 +435,12 @@ interface CustomConfig {
    idleTimeoutSecs?: number     // default: 30
 }
 
+interface AttachedDatabaseSpec {
+   databasePath: string  // Path relative to app config directory
+   schemaName: string    // Schema name for accessing tables (e.g., 'orders')
+   mode: 'readOnly' | 'readWrite'
+}
+
 interface SqliteError {
    code: string
    message: string

api-iife.js

@@ -94,7 +94,7 @@ times is safe (already-applied migrations are skipped).
 
 ### Attached Databases
 
-Attach other SQLite databases to enable cross-database queries. Attachments are
+Attach other SQLite databases to enable cross-database queries. Attached databases are
 connection-scoped and automatically detached when the guard is dropped.
 
 ```rust
@@ -175,8 +175,8 @@ returned to pool on drop.
 
 | Function | Description |
 | -------- | ----------- |
-| `acquire_reader_with_attached(db, specs)` | Acquire read connection with attached databases |
-| `acquire_writer_with_attached(db, specs)` | Acquire writer connection with attached databases |
+| `acquire_reader_with_attached(db, specs)` | Acquire read connection with attached database(s) |
+| `acquire_writer_with_attached(db, specs)` | Acquire writer connection with attached database(s) |
 
 Returns `AttachedConnection` or `AttachedWriteGuard` respectively. Both guards
 deref to `SqliteConnection` and automatically detach databases on drop.

crates/sqlx-sqlite-conn-mgr/README.md

@@ -30,9 +30,9 @@ pub enum AttachedMode {
    ReadWrite,
 }
 
-/// Guard holding a read connection with attached databases
+/// Guard holding a read connection with attached database(s)
 ///
-/// **Important**: Call `detach_all()` before dropping to properly clean up attached databases.
+/// **Important**: Call `detach_all()` before dropping to properly clean up attached database(s).
 /// Without explicit cleanup, attached databases persist on the pooled connection until
 /// it's eventually closed. Derefs to `SqliteConnection` for executing queries.
 #[must_use = "if unused, the attached connection and locks are immediately dropped"]
@@ -100,7 +100,7 @@ impl Drop for AttachedReadConnection {
    }
 }
 
-/// Guard holding a write connection with attached databases
+/// Guard holding a write connection with attached database(s)
 ///
 /// **Important**: Call `detach_all()` before dropping to properly clean up attached databases.
 /// Without explicit cleanup, attached databases persist on the pooled connection until
@@ -189,7 +189,7 @@ fn is_valid_schema_name(name: &str) -> bool {
       && !name.chars().next().unwrap().is_ascii_digit()
 }
 
-/// Acquire a read connection with attached databases
+/// Acquire a read connection with attached database(s)
 ///
 /// This function:
 /// 1. Acquires a read connection from the main database's read pool
@@ -231,7 +231,7 @@ pub async fn acquire_reader_with_attached(
    for spec in &specs {
       let path = spec.database.path_str();
       if !seen_paths.insert(path.clone()) {
-         return Err(Error::DuplicateDatabaseAttachment(path));
+         return Err(Error::DuplicateAttachedDatabase(path));
       }
    }
 
@@ -261,7 +261,7 @@ pub async fn acquire_reader_with_attached(
    Ok(AttachedReadConnection::new(conn, Vec::new(), schema_names))
 }
 
-/// Acquire a write connection with attached databases
+/// Acquire a write connection with attached database(s)
 ///
 /// This function:
 /// 1. Acquires the write connection from the main database
@@ -320,7 +320,7 @@ pub async fn acquire_writer_with_attached(
    let mut seen_paths = HashSet::new();
    for (path, _) in &db_entries {
       if !seen_paths.insert(path.as_str()) {
-         return Err(Error::DuplicateDatabaseAttachment(path.clone()));
+         return Err(Error::DuplicateAttachedDatabase(path.clone()));
       }
    }
 
@@ -517,19 +517,21 @@ mod tests {
          .fetch_one(&mut *conn)
          .await
          .unwrap();
+
       let value1: String = row1.get(0);
       assert_eq!(value1, "test_data");
 
       let row2 = sqlx::query("SELECT value FROM db2.db2 LIMIT 1")
          .fetch_one(&mut *conn)
          .await
          .unwrap();
+
       let value2: String = row2.get(0);
       assert_eq!(value2, "test_data");
    }
 
    #[tokio::test]
-   async fn test_readwrite_attachment_holds_writer_lock() {
+   async fn test_attached_database_in_readwrite_mode_holds_writer_lock() {
       let temp_dir = TempDir::new().unwrap();
       let main_db = create_test_db("main.db", &temp_dir).await;
       let other_db = create_test_db("other.db", &temp_dir).await;
@@ -541,7 +543,7 @@ mod tests {
       }];
 
       // Acquire writer with attached database (holds other_db's writer)
-      let _attached_writer = acquire_writer_with_attached(&main_db, specs).await.unwrap();
+      let _guard = acquire_writer_with_attached(&main_db, specs).await.unwrap();
 
       // Try to acquire other_db's writer directly - should block/timeout
       let acquire_result = tokio::time::timeout(
@@ -571,7 +573,7 @@ mod tests {
 
       // Acquire and drop
       {
-         let _attached_writer = acquire_writer_with_attached(&main_db, specs).await.unwrap();
+         let _ = acquire_writer_with_attached(&main_db, specs).await.unwrap();
          // Dropped at end of scope
       }
 
@@ -646,7 +648,7 @@ mod tests {
    }
 
    #[tokio::test]
-   async fn test_attached_sorting_prevents_deadlock() {
+   async fn test_sorting_attached_databases_prevents_deadlock() {
       let temp_dir = TempDir::new().unwrap();
       let main_db = create_test_db("main.db", &temp_dir).await;
       let db_a = create_test_db("a.db", &temp_dir).await;
@@ -675,7 +677,7 @@ mod tests {
    }
 
    #[tokio::test]
-   async fn test_cross_database_attachment_no_deadlock() {
+   async fn test_attaching_same_databases_in_different_order_concurrently_no_deadlock() {
       // This test verifies the fix for the deadlock scenario:
       // Thread 1: main=A, attach B
       // Thread 2: main=B, attach A
@@ -765,7 +767,7 @@ mod tests {
    }
 
    #[tokio::test]
-   async fn test_duplicate_database_rejected() {
+   async fn test_duplicate_attached_database_rejected() {
       let temp_dir = TempDir::new().unwrap();
       let main_db = create_test_db("main.db", &temp_dir).await;
       let other_db = create_test_db("other.db", &temp_dir).await;
@@ -786,8 +788,8 @@ mod tests {
 
       let result = acquire_writer_with_attached(&main_db, specs).await;
       assert!(
-         matches!(result, Err(Error::DuplicateDatabaseAttachment(_))),
-         "Should reject duplicate database attachment"
+         matches!(result, Err(Error::DuplicateAttachedDatabase(_))),
+         "Should reject duplicate attached database"
       );
    }
 
@@ -805,7 +807,7 @@ mod tests {
 
       let result = acquire_writer_with_attached(&main_db, specs).await;
       assert!(
-         matches!(result, Err(Error::DuplicateDatabaseAttachment(_))),
+         matches!(result, Err(Error::DuplicateAttachedDatabase(_))),
          "Should reject attaching main database to itself"
       );
    }

crates/sqlx-sqlite-conn-mgr/src/attached.rs

@@ -69,13 +69,11 @@ pub struct SqliteDatabase {
 impl SqliteDatabase {
    /// Get the database file path as a string
    ///
-   /// Used internally for ATTACH DATABASE statements
+   /// Used internally (crate-private) for ATTACH DATABASE statements
    pub(crate) fn path_str(&self) -> String {
       self.path.to_string_lossy().to_string()
    }
-}
 
-impl SqliteDatabase {
    /// Connect to a SQLite database
    ///
    /// If the database is already connected, returns the existing connection.

crates/sqlx-sqlite-conn-mgr/src/database.rs

@@ -33,6 +33,8 @@ pub enum Error {
    InvalidSchemaName(String),
 
    /// Attempted to attach the same database multiple times
-   #[error("Database '{0}' appears multiple times in attachment list (would cause deadlock)")]
-   DuplicateDatabaseAttachment(String),
+   #[error(
+      "Database '{0}' appears multiple times in attached database list (would cause deadlock)"
+   )]
+   DuplicateAttachedDatabase(String),
 }