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我正在尝试为每个池连接准备语句。

我基本上想做的是：

const SQL_1: &str = "SELECT foo FROM bar WHERE baz = $1";

在池连接创建期间，为每个新连接运行：

let stmt = conn.prepare(SQL_1);
__some_magic_to_store_at_connection__(SQL_1, stmt);

在处理程序中时：

let conn = my_pool.get().unwrap();
let stmt = __some_magic_to_get_the_stmt__(conn, SQL_1);

现在怎么办？到目前为止，我想出了这个：

const SQL_1: &str = "SELECT foo FROM bar WHERE baz = $1";

// Customized connection which will run all required PREPARE
#[derive(Debug)]
pub struct PreparedConnection {}

// Trait implementation for customized connection
impl r2d2::CustomizeConnection<r2d2_postgres::postgres::Client, r2d2_postgres::postgres::Error> for PreparedConnection {

    // Run all statement preparators
    fn on_acquire(&self, p_conn: &mut r2d2_postgres::postgres::Client) -> Result<(), r2d2_postgres::postgres::Error> {
        
        // Prepare it
        let stmt = p_conn.prepare(SQL_1);
        
        // Now ... what?
        
        // Done
        Ok(())
    }

    // Noting to do on release
    fn on_release(&self, _: r2d2_postgres::postgres::Client) {}
}

所以我需要以某种方式将语句存储在连接中。稍后再取回。我看到有一个可用于池连接的 extensions() 东西，但我正在努力让它工作。

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