4

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);
}

问题是编译器抱怨一些缺失的特征边界 forC和&Cat.execute(&conn)我不太确定如何将它们放在where子句中，可能还有一种我不知道的更简单的方法。任何提示都非常受欢迎！

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！

4

2 回答 2

4

我终于通过定义以下特征界限来解决了！

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);

Table基本上，您需要为和提供一对额外的界限Insertable。如果可以直接从 中获取表会很好Insertable，以避免在函数定义中使用其他类型，但我可以使用它:)

于 2019-10-28T12:22:20.903 回答
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);
}
于 2020-09-05T23:21:22.560 回答