我在向量中分组项目时遇到了问题。到目前为止，我所拥有的是以下内容（简化）：

rs #[derive(Debug, Clone, Queryable, Serialize, Deserialize)]
pub(crate) struct User {   
    id: i64,   
    firstname: String,   
    lastname: String 
}

#[derive(Debug, Clone, Queryable, Serialize, Deserialize)]
pub(crate) struct Item {
    id: i64,
    name: String,
    owner: User,
    owner_id: i64
}

let mut _items: Vec<Item> = items::dsl::items
.inner_join(users::table.on(users::id.eq(items::owner_id))
.select((
    items::id,
    items::name,
    (
        users::id
        users::firstname,
        users::lastname,
    ),
    items::owner_id,
))
.order_by(items::owner_id)
.load::<Item>(&mut conn)?;

这个方法有效，我得到了一个项目向量.
现在我想要的是一个向量，其元素的形式为：

#[derive(Debug, Clone, Queryable, Serialize, Deserialize)]
pub(crate) struct UserWithItems {
    id: i64,
    firstname: String,
    lastname: String,
    items: Vec<SmallerItem>
}

where

#[derive(Debug, Clone, Serialize, Deserialize)]
pub(crate) struct SmallerItem {
    id: i64,
    name: String,
    owner_id: i64
}

即每个条目对应于用户的向量，并且这些用户中的每一个具有包含其所有项的向量。
我尝试跟随https://stackoverflow.com/a/69663425/19389450：

let res: Vec<UserWithItems> = _items
        .iter()
        .group_by(|x| (x.user_id, x.firstname, x.lastname))
        .into_iter()
        .map(|((id, firstname, lastname), group)| UserWithItems {
            id, 
            firstname, 
            lastname,
            items: Some(group.map(|i| SmallerItem {
                id: i.id,
                name: i.name,
                owner_id: i.owner_id
            }).collect()),
        })
        .collect();

但我得到的错误：

|
126 |             .group_by(|x| (x.user_id, x.firstname, x.lastname)
    |              ^^^^^^^^ method cannot be called on std::slice::Iter<'_, Item> due to
    |                       unsatisfied trait bounds
    |
   ::: /----/.rustup/toolchains/stable-aarch64-apple-
    |  darwin/lib/rustlib/src/rust/library/core/src/slice/iter.rs:66:1
    |
66  | pub struct Iter<'a, T: 'a> {
    | --------------------------
    | |
    | doesn't satisfy `std::slice::Iter<'_, Item>: Table`
    | doesn't satisfy `std::slice::Iter<'_, Item>: diesel::QueryDsl`
    |
    = note: the following trait bounds were not satisfied:
            `std::slice::Iter<'_, Item>: Table`
            which is required by `std::slice::Iter<'_, Item>: diesel::QueryDsl`
            `&std::slice::Iter<'_, Item>: Table`
            which is required by `&std::slice::Iter<'_, Item>: diesel::QueryDsl`
            `&mut std::slice::Iter<'_, Item>: Table`
            which is required by `&mut std::slice::Iter<'_, Item>: diesel::QueryDsl`

[package]
name = "items"
version = "0.1.0"
authors = ["email"]
edition = "2021"

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

[dependencies]
actix = "0.13.0"
actix-web = "4.2.1"
chrono = { version = "0.4.22", features = ["serde"]}
diesel = { version = "2.0.2", features = ["r2d2", "mysql", "chrono"] }
diesel_migrations = "2.0.0"
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0"
dotenvy = "0.15.6"
env_logger = "0.10.0"
jwt = "0.16.0"
sha2 = "0.10.6"
hmac = "0.12.1"
derive_more = "0.99.17"
argon2 = "0.4.1"`your text`
rand_core = { version = "0.6", features = ["std"] }
futures-util = "0.3.25"
validator = { version = "0.16", features = ["derive"] }
thiserror = "1.0.37"
redis = { version = "0.22.1", features = ["r2d2"] }