Rust+Actix-Web构建REST API遇LoadConnection trait未实现错误求助

我正在用Rust和Actix-Web构建REST API，使用Diesel 2.0.3操作PostgreSQL数据库，但运行时持续报错。相关代码如下：

main.rs（连接池配置） #

#[actix_web::main]
async fn main() -> std::io::Result<()> {
// Loading .env into environment variable.
dotenv::dotenv().ok();

env_logger::init_from_env(env_logger::Env::new().default_filter_or("info"));

// set up database connection pool
let database_url = std::env::var("DATABASE_URL").expect("DATABASE_URL");
let manager = ConnectionManager::<PgConnection>::new(database_url);
let pool: DbPool = r2d2::Pool::builder()
    .test_on_check_out(true)
    .build(manager)
    .expect("Could not build connection pool");
let port = std::env::var("PORT").expect("$PORT is not set.");
HttpServer::new(move || {
    App::new()
        .app_data(web::Data::new(pool.clone()))
        .wrap(middleware::Logger::default())
        .route("/", web::get().to(|| async { "Actix REST API" }))
        .service(handlers::common::houses::index)
})
.bind(("0.0.0.0", port.parse().unwrap()))?
.run()
.await
}

schema定义 #

diesel::table! {
  houses (id) {
    id -> Int4,
    user_id -> Varchar,
    street -> Varchar,
    figure -> Varchar,
    floor -> Varchar,
    create_date -> Timestamp,
    update_date -> Timestamp,
    is_deleted -> Bool,
  }
}

House模型 #

#[derive(Debug, Serialize, Deserialize, Queryable)]
  pub struct House {
  pub id: i32,
  pub user_id: String,
  pub street: String,
  pub figure: String,
  pub floor: String,
  pub create_date: chrono::NaiveDateTime,
  pub update_date: chrono::NaiveDateTime,
  pub is_deleted: bool,
}

handler代码 #

#[get("/houses")]
async fn index(pool: web::Data<DbPool>) -> Result<HttpResponse, Error> {
  let houses = web::block(move || {
    let conn = &pool.get()?;
    find_all(&conn)
  })
  .await?
  .map_err(actix_web::error::ErrorInternalServerError)?;

  Ok(HttpResponse::Ok().json(houses))
}

fn find_all(conn: &PgConnection) -> Result<Vec<House>, DbError> {
   use crate::schemas::common::houses::houses::dsl::*;

   let items =houses.load::<House>(&mut conn)?;
   Ok(items)
}

依赖配置 #

[dependencies]
actix-web = "4"
chrono = { version = "0.4.19", features = ["serde"] }
diesel = { version = "2.0.3", features = ["postgres", "r2d2", "chrono"] }
dotenv = "0.15.0"
env_logger = "0.10.0"
serde = { version = "1.0.136", features = ["derive"] }
serde_json = "1.0"

错误信息 #

error[E0277]: the trait bound `&diesel::PgConnection: LoadConnection` is not satisfied src\handlers\common\houses.rs:25:37
     | 25   |     let items =houses.load::<House>(&mut conn)?;
     |                       ----          -^^^^^^^^
     |                       |             | the trait `LoadConnection` is not implemented for `&diesel::PgConnection` help: consider removing the leading `&`-reference required by a bound introduced by this call
     | note: required for `table` to implement `LoadQuery<'_, &diesel::PgConnection, House>` note: required by a bound in `diesel::RunQueryDsl::load`

这个错误的核心是Diesel 2.0对连接的借用规则做了调整，代码中出现了多层引用的冲突问题：

1. index函数中，pool.get()返回连接实例，你取了它的不可变引用&conn传给find_all
2. find_all里又试图对这个不可变引用加可变借用&mut conn，导致类型不匹配，不符合LoadConnection trait的要求

修改步骤如下：

1. 调整连接获取方式 #

在index的web::block闭包中，直接获取可变连接的所有权：

#[get("/houses")]
async fn index(pool: web::Data<DbPool>) -> Result<HttpResponse, Error> {
  let houses = web::block(move || {
    let mut conn = pool.get()?; // 获取可变连接的所有权
    find_all(&mut conn)
  })
  .await?
  .map_err(actix_web::error::ErrorInternalServerError)?;

  Ok(HttpResponse::Ok().json(houses))
}

2. 修改find_all函数的参数和调用 #

让find_all接收可变连接引用，调用load时直接传递该引用：

fn find_all(conn: &mut PgConnection) -> Result<Vec<House>, DbError> {
   use crate::schemas::common::houses::houses::dsl::*;

   let items = houses.load::<House>(conn)?; // 直接传可变引用，无需额外加&mut
   Ok(items)
}

原因说明 #

Diesel 2.0中，load方法需要的是可变的连接引用，你之前的代码把连接变成了&mut &PgConnection类型，完全不符合约束要求。通过调整连接的获取和传递方式，让conn成为&mut PgConnection类型，就能满足LoadConnection trait的实现条件。

内容的提问来源于stack exchange，提问作者A G