为泛型类型编写 Diesel CRUD 操作

Posted 2023-03-21

【中文标题】为泛型类型编写 Diesel CRUD 操作

【英文标题】：Writing Diesel CRUD operations for generic types

【发布时间】：2020-02-23 14:33:20

【问题描述】：

我正在尝试编写一个 Rust crate，它在使用 Diesel 创建简单的 CRUD 操作时从用户那里删除一些样板代码

例如，如果您有一台像这样的柴油机Insertable：

#[derive(Insertable)]
#[table_name = "users"]
pub struct UserCreate<'a> 
    pub email: String,
    pub hash: &'a [u8],
    pub first_name: Option<String>,
    pub family_name: Option<String>,

我希望 crate 用户只写create<UserCreate>(model, pool)，将结构字段插入数据库行。

为此，我编写了以下函数签名（例如简化）：

fn create<'a, C: 'a>(model: C, pool: DBPool)
where
    C: diesel::Identifiable,
    &'a C: diesel::Insertable<C::Table>,

    let conn = pool.get().unwrap();
    diesel::insert_into(C::table())
        .values(&model)
        .execute(&conn);

问题是编译器在.execute(&conn) 抱怨C 和&C 缺少一些特征边界，我不太确定如何将它们放在where 子句中，可能还有一个更简单的我不知道这样做的方式。非常欢迎任何提示！

编译器输出：

error[E0277]: the trait bound `<<C as diesel::associations::HasTable>::Table as diesel::QuerySource>::FromClause: diesel::query_builder::QueryFragment<_>` is not satisfied
  --> database/src/users/models.rs:46:10
   |
46 |         .execute(&conn);
   |          ^^^^^^^ the trait `diesel::query_builder::QueryFragment<_>` is not implemented for `<<C as diesel::associations::HasTable>::Table as diesel::QuerySource>::FromClause`
   |
   = help: the following implementations were found:
             <&'a T as diesel::query_builder::QueryFragment<DB>>
   = note: required because of the requirements on the impl of `diesel::query_builder::QueryFragment<_>` for `diesel::query_builder::InsertStatement<<C as diesel::associations::HasTable>::Table, <&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values>`
   = note: required because of the requirements on the impl of `diesel::query_dsl::load_dsl::ExecuteDsl<_, _>` for `diesel::query_builder::InsertStatement<<C as diesel::associations::HasTable>::Table, <&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values>`

error[E0277]: the trait bound `<&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values: diesel::query_builder::QueryFragment<_>` is not satisfied
  --> database/src/users/models.rs:46:10
   |
46 |         .execute(&conn);
   |          ^^^^^^^ the trait `diesel::query_builder::QueryFragment<_>` is not implemented for `<&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values`
   |
   = help: the following implementations were found:
             <&'a T as diesel::query_builder::QueryFragment<DB>>
   = note: required because of the requirements on the impl of `diesel::query_builder::QueryFragment<_>` for `diesel::query_builder::InsertStatement<<C as diesel::associations::HasTable>::Table, <&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values>`
   = note: required because of the requirements on the impl of `diesel::query_dsl::load_dsl::ExecuteDsl<_, _>` for `diesel::query_builder::InsertStatement<<C as diesel::associations::HasTable>::Table, <&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values>`

error[E0277]: the trait bound `<&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values: diesel::insertable::CanInsertInSingleQuery<_>` is not satisfied
  --> database/src/users/models.rs:46:10
   |
46 |         .execute(&conn);
   |          ^^^^^^^ the trait `diesel::insertable::CanInsertInSingleQuery<_>` is not implemented for `<&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values`
   |
   = help: the following implementations were found:
             <&'a T as diesel::insertable::CanInsertInSingleQuery<DB>>
   = note: required because of the requirements on the impl of `diesel::query_builder::QueryFragment<_>` for `diesel::query_builder::InsertStatement<<C as diesel::associations::HasTable>::Table, <&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values>`
   = note: required because of the requirements on the impl of `diesel::query_dsl::load_dsl::ExecuteDsl<_, _>` for `diesel::query_builder::InsertStatement<<C as diesel::associations::HasTable>::Table, <&C as diesel::Insertable<<C as diesel::associations::HasTable>::Table>>::Values>`

error: aborting due to 3 previous errors

非常感谢！

【参考方案1】：

我终于通过定义以下 trait bounds 解决了！

fn create<C, T>(model: C, pool: DBPool)
where
    T: diesel::associations::HasTable,
    <T::Table as diesel::QuerySource>::FromClause:
        diesel::query_builder::QueryFragment<diesel::pg::Pg>,
    C: diesel::Insertable<T::Table>,
    C::Values: diesel::insertable::CanInsertInSingleQuery<diesel::pg::Pg>
        + diesel::query_builder::QueryFragment<diesel::pg::Pg>,

    let conn = pool.get().unwrap();

    diesel::insert_into(T::table())
        .values(model)
        .execute(&conn);


create::<UserCreate, users::table>(user, pool);

基本上，Table 和 Insertable 需要一对额外的界限。如果可以直接从Insertable 获取表就好了，以避免在函数定义中使用其他类型，但我可以使用它:)

【讨论】：

嗨，如果我想用get_result 替换excute，我需要实现什么特征。谢谢@codearm

【参考方案2】：

接受的答案是正确的，但可以通过使用更高的界限来减少冗长

use diesel::query_builder::InsertStatement;
use diesel::query_dsl::methods::ExecuteDsl;

pub fn insert_into_table<T, M>(conn: &Pgconnection, table: T, records: M)
where
    T: Table,
    M: diesel::Insertable<T>,
    InsertStatement<T, M::Values>: ExecuteDsl<PgConnection>,

    diesel::insert_into(table)
        .values(records)
        .execute(conn);