Rust Diesel中load方法类型参数不兼容问题解决

我尝试使用Diesel编写一个简单查询，匹配给定凭证的用户。查阅Diesel文档后，我认为需要在load方法使用的结构体上添加Queryable宏，但出现了类型不兼容错误。

编译错误信息 #

error[E0277]: the trait bound `(Integer, Text, Text, Text, diesel::sql_types::Timestamp): load_dsl::private::CompatibleType<dal::models::users::UserView<'_>, _>` is not satisfied
    --> src/services/user.rs:16:27
     |
16   |         .load::<UserView>(connection);
     |          ----             ^^^^^^^^^^ the trait `load_dsl::private::CompatibleType<dal::models::users::UserView<'_>, _>` is not implemented for `(Integer, Text, Text, Text, diesel::sql_types::Timestamp)`
     |          |
     |          required by a bound introduced by this call
     |
     = help: the following other types implement trait `load_dsl::private::CompatibleType<U, DB>`:
               (ST0, ST1)
               (ST0, ST1, ST2)
               (ST0, ST1, ST2, ST3)
               (ST0, ST1, ST2, ST3, ST4)
               (ST0, ST1, ST2, ST3, ST4, ST5)
               (ST0, ST1, ST2, ST3, ST4, ST5, ST6)
               (ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7)
               (ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8)
             and 24 others
     = note: required for `SelectStatement<FromClause<table>, DefaultSelectClause<FromClause<table>>, NoDistinctClause, WhereClause<Grouped<And<Grouped<Eq<username, ...>>, ...>>>>` to implement `LoadQuery<'_, _, dal::models::users::UserView<'_>>`
     = note: the full type name has been written to '/home/pop/Documents/code/PECI-05-2022-23/auth/target/debug/deps/auth-955be7812e7a3b43.long-type-7337842634617248670.txt'
note: required by a bound in `diesel::RunQueryDsl::load`
    --> /home/pop/.cargo/registry/src/index.crates.io-6f17d22bba15001f/diesel-2.0.3/src/query_dsl/mod.rs:1499:15
     |
1499 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::load`

error[E0277]: the trait bound `dal::models::users::UserView<'_>: FromSqlRow<_, _>` is not satisfied
    --> src/services/user.rs:16:27
     |
16   |         .load::<UserView>(connection);
     |          ----             ^^^^^^^^^^ the trait `FromSqlRow<_, _>` is not implemented for `dal::models::users::UserView<'_>`
     |          |
     |          required by a bound introduced by this call
     |
     = help: the following other types implement trait `FromSqlRow<ST, DB>`:
               <(T1, T0) as FromSqlRow<(ST1, Untyped), __DB>>
               <(T1, T2, T0) as FromSqlRow<(ST1, ST2, Untyped), __DB>>
               <(T1, T2, T3, T0) as FromSqlRow<(ST1, ST2, ST3, Untyped), __DB>>
               <(T1, T2, T3, T4, T0) as FromSqlRow<(ST1, ST2, ST3, ST4, Untyped), __DB>>
               <(T1, T2, T3, T4, T5, T0) as FromSqlRow<(ST1, ST2, ST3, ST4, ST5, Untyped), __DB>>
               <(T1, T2, T3, T4, T5, T6, T0) as FromSqlRow<(ST1, ST2, ST3, ST4, ST5, ST6, Untyped), __DB>>
               <(T1, T2, T3, T4, T5, T6, T7, T0) as FromSqlRow<(ST1, ST2, ST3, ST4, ST5, ST6, ST7, Untyped), __DB>>
               <(T1, T2, T3, T4, T5, T6, T7, T8, T0) as FromSqlRow<(ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8, Untyped), __DB>>
             and 23 others
     = note: required for `SelectStatement<FromClause<table>, DefaultSelectClause<FromClause<table>>, NoDistinctClause, WhereClause<Grouped<And<Grouped<Eq<username, ...>>, ...>>>>` to implement `LoadQuery<'_, _, dal::models::users::UserView<'_>>`
     = note: the full type name has been written to '/home/pop/Documents/code/PECI-05-2022-23/auth/target/debug/deps/auth-955be7812e7a3b43.long-type-7337842634617248670.txt'
note: required by a bound in `diesel::RunQueryDsl::load`
    --> /home/pop/.cargo/registry/src/index.crates.io-6f17d22bba15001f/diesel-2.0.3/src/query_dsl/mod.rs:1499:15
     |
1499 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::load`

For more information about this error, try `rustc --explain E0277`.
warning: `auth` (bin "auth") generated 3 warnings
error: could not compile `auth` (bin "auth") due to 2 previous errors; 3 warnings emitted

相关代码 #

登录函数 #

pub fn login(user_credentials: UserLogin) -> bool {
    use crate::dal::schema::users::dsl::*;
    let connection = &mut get_connection();
    let result = users
        .filter(username.eq(user_credentials.username))
        .filter(password.eq(user_credentials.password))
        .load::<UserView>(connection);

    match result {
        Ok(query_res) => return query_res.len() == 1,
        Err(_) => return false
    }
}

初始的UserView结构体 #

#[derive(Queryable)]
pub struct UserView<'a> {
    pub user_id: i32,
    pub username: &'a str,
    pub email: &'a str,
    pub created_on: SystemTime
}

Users表Schema定义 #

diesel::table! {
    users (user_id) {
        user_id -> Int4,
        username -> Varchar,
        password -> Varchar,
        email -> Varchar,
        created_on -> Timestamp,
    }
}

Cargo.toml依赖配置 #

[package]
name = "auth"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
rocket = { version = "0.5.0-rc.2", features= ["json"]}
diesel = { version = "2.0.1", features = ["postgres", "chrono"] }
dotenv = "0.15.0"
chrono = "^0"
serde = { version = "1.0.117", features = ["derive"] }
serde_json = "1.0.59"

问题解决 #

将UserView结构体修改为以下形式后问题解决。错误原因是之前误以为load可以只选择指定字段，但当前查询是查询users表的所有字段，因此结构体必须包含表中所有列，同时不能使用引用类型（如&str），需要使用拥有所有权的类型（如String）：

#[derive(Queryable)]
pub struct UserView {
    pub user_id : i32,
    pub username : String,
    pub password : String,
    pub email: String,
    pub created_on: SystemTime,
}

内容的提问来源于stack exchange，提问作者João Machado