feat: Support for lazy database connections (#2268)

Add support for creating DB connection pools without establishing
connections up front. This is already supported by SQLx via the
[Pool::connect_lazy](https://docs.rs/sqlx/latest/sqlx/struct.Pool.html#method.connect_lazy)
method.

This PR adds a new `connect_lazy` option to `database::ConnectOptions`.
If set to `true`, the SQLx `Pool` will be created using the
`Pool::connect_lazy_with` method; otherwise, the `Pool::connect_with`
method will be used (e.g., the existing behavior). This "lazy" behavior
is implemented for each DB variant (Postgres/MySQL/SQLite).

This was discussed previously
[here](https://github.com/SeaQL/sea-orm/discussions/1645), but it
appears support was never added to SEA ORM directly.

src/database/mod.rs

@@ -63,10 +63,15 @@ pub struct ConnectOptions {
     /// Schema search path (PostgreSQL only)
     pub(crate) schema_search_path: Option<String>,
     pub(crate) test_before_acquire: bool,
+    /// Only establish connections to the DB as needed. If set to `true`, the db connection will
+    /// be created using SQLx's [connect_lazy](https://docs.rs/sqlx/latest/sqlx/struct.Pool.html#method.connect_lazy)
+    /// method.
+    pub(crate) connect_lazy: bool,
 }
 
 impl Database {
-    /// Method to create a [DatabaseConnection] on a database
+    /// Method to create a [DatabaseConnection] on a database. This method will return an error
+    /// if the database is not available.
     #[instrument(level = "trace", skip(opt))]
     pub async fn connect<C>(opt: C) -> Result<DatabaseConnection, DbErr>
     where
@@ -157,6 +162,7 @@ impl ConnectOptions {
             sqlcipher_key: None,
             schema_search_path: None,
             test_before_acquire: true,
+            connect_lazy: false,
         }
     }
 
@@ -297,4 +303,16 @@ impl ConnectOptions {
         self.test_before_acquire = value;
         self
     }
+
+    /// If set to `true`, the db connection pool will be created using SQLx's
+    /// [connect_lazy](https://docs.rs/sqlx/latest/sqlx/struct.Pool.html#method.connect_lazy) method.
+    pub fn connect_lazy(&mut self, value: bool) -> &mut Self {
+        self.connect_lazy = value;
+        self
+    }
+
+    /// Get whether DB connections will be established when the pool is created or only as needed.
+    pub fn get_connect_lazy(&self) -> bool {
+        self.connect_lazy
+    }
 }

src/driver/sqlx_mysql.rs

@@ -61,15 +61,21 @@ impl SqlxMySqlConnector {
                 );
             }
         }
-        match options.sqlx_pool_options().connect_with(opt).await {
-            Ok(pool) => Ok(DatabaseConnection::SqlxMySqlPoolConnection(
+        let pool = if options.connect_lazy {
+            options.sqlx_pool_options().connect_lazy_with(opt)
+        } else {
+            options
+                .sqlx_pool_options()
+                .connect_with(opt)
+                .await
+                .map_err(sqlx_error_to_conn_err)?
+        };
+        Ok(DatabaseConnection::SqlxMySqlPoolConnection(
             SqlxMySqlPoolConnection {
                 pool,
                 metric_callback: None,
             },
-            )),
-            Err(e) => Err(sqlx_error_to_conn_err(e)),
-        }
+        ))
     }
 }
 

src/driver/sqlx_postgres.rs

@@ -65,6 +65,7 @@ impl SqlxPostgresConnector {
             .schema_search_path
             .as_ref()
             .map(|schema| format!("SET search_path = {schema}"));
+        let lazy = options.connect_lazy;
         let mut pool_options = options.sqlx_pool_options();
         if let Some(sql) = set_search_path_sql {
             pool_options = pool_options.after_connect(move |conn, _| {
@@ -76,15 +77,20 @@ impl SqlxPostgresConnector {
                 })
             });
         }
-        match pool_options.connect_with(opt).await {
-            Ok(pool) => Ok(DatabaseConnection::SqlxPostgresPoolConnection(
+        let pool = if lazy {
+            pool_options.connect_lazy_with(opt)
+        } else {
+            pool_options
+                .connect_with(opt)
+                .await
+                .map_err(sqlx_error_to_conn_err)?
+        };
+        Ok(DatabaseConnection::SqlxPostgresPoolConnection(
             SqlxPostgresPoolConnection {
                 pool,
                 metric_callback: None,
             },
-            )),
-            Err(e) => Err(sqlx_error_to_conn_err(e)),
-        }
+        ))
     }
 }
 

src/driver/sqlx_sqlite.rs

@@ -66,11 +66,21 @@ impl SqlxSqliteConnector {
                 );
             }
         }
+
         if options.get_max_connections().is_none() {
             options.max_connections(1);
         }
-        match options.sqlx_pool_options().connect_with(opt).await {
-            Ok(pool) => {
+
+        let pool = if options.connect_lazy {
+            options.sqlx_pool_options().connect_lazy_with(opt)
+        } else {
+            options
+                .sqlx_pool_options()
+                .connect_with(opt)
+                .await
+                .map_err(sqlx_error_to_conn_err)?
+        };
+
         let pool = SqlxSqlitePoolConnection {
             pool,
             metric_callback: None,
@@ -84,9 +94,6 @@ impl SqlxSqliteConnector {
 
         Ok(DatabaseConnection::SqlxSqlitePoolConnection(pool))
     }
-            Err(e) => Err(sqlx_error_to_conn_err(e)),
-        }
-    }
 }
 
 impl SqlxSqliteConnector {