我正在尝试使用通用柴油机功能来缩减重复性任务，例如根据主键删除行。
我得到的通用行插入工作相对较快，但删除查询似乎很难。我尝试使用find()和filter()解决它。我也咨询了类似的主题1和2没有成功。
finduse diesel::prelude::*;
use diesel::query_dsl::methods::FindDsl;
use std::error::Error;
pub struct DB {
conn: SqliteConnection,
}
impl DB {
pub fn remove_row<'a, T>(&self, table: T, pk: &'a str) -> Result<(), Box<Error>>
where
T: FindDsl<&'a str>,
<T as FindDsl<&'a str>>::Output: diesel::Identifiable,
<T as FindDsl<&'a str>>::Output: diesel::associations::HasTable,
{
diesel::delete(table.find(pk)).execute(&self.conn)?;
Ok(())
}
}
这会导致以下错误，我根本无法解释：
error[E0275]: overflow evaluating the requirement `_: std::marker::Sized`
--> src/db/mod.rs:103:3
|
103 | diesel::delete (table.find (pk)) .execute (&self.conn) ?;
| ^^^^^^^^^^^^^^
|
= help: consider adding a `#![recursion_limit="128"]` attribute to your crate
= note: required because of the requirements on the impl of `diesel::query_dsl::filter_dsl::FilterDsl<_>` for `<<<T as diesel::query_dsl::filter_dsl::FindDsl<&'a str>>::Output as diesel::associations::HasTable>::Table as diesel::query_builder::AsQuery>::Query`
= note: required because of the requirements on the impl of `diesel::query_builder::IntoUpdateTarget` for `<T as diesel::query_dsl::filter_dsl::FindDsl<&'a str>>::Output`
= note: required by `diesel::delete`
filter()use diesel::prelude::*;
use diesel::query_dsl::methods::FilterDsl;
use std::error::Error;
pub struct DB {
conn: SqliteConnection,
}
impl DB {
pub fn remove_row<T>(&self, table: T, pk: &str) -> Result<(), Box<Error>>
where
T: FilterDsl<bool>,
<T as FilterDsl<bool>>::Output: diesel::Identifiable,
<T as FilterDsl<bool>>::Output: diesel::associations::HasTable,
{
diesel::delete(table.filter(id.eq(pk))).execute(&self.conn)?;
Ok(())
}
}
除了上一个错误之外，还有一条关于id在数据结构中未知的错误消息。我可以考虑一个缺失的特性，它保证了那一行的存在，但我还没有发现任何关于这种行为的信息。
error[E0425]: cannot find value `id` in this scope
--> src/db/mod.rs:117:33
|
117 | diesel::delete (table.filter (id.eq (pk))) .execute (&self.conn) ?;
| ^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
|
4 | use crate::db::schema::events::columns::id;
|
4 | use crate::db::schema::ignored_events::columns::id;
|
4 | use crate::db::schema::locations::columns::id;
|
4 | use std::process::id;
error[E0275]: overflow evaluating the requirement `_: std::marker::Sized`
--> src/db/mod.rs:117:3
|
117 | diesel::delete (table.filter (id.eq (pk))) .execute (&self.conn) ?;
| ^^^^^^^^^^^^^^
|
= help: consider adding a `#![recursion_limit="128"]` attribute to your crate
= note: required because of the requirements on the impl of `diesel::query_dsl::filter_dsl::FilterDsl<_>` for `<<<T as diesel::query_dsl::filter_dsl::FilterDsl<bool>>::Output as diesel::associations::HasTable>::Table as diesel::query_builder::AsQuery>::Query`
= note: required because of the requirements on the impl of `diesel::query_builder::IntoUpdateTarget` for `<T as diesel::query_dsl::filter_dsl::FilterDsl<bool>>::Output`
= note: required by `diesel::delete`
泛型并不容易。像Diesel这样的高度通用系统中的泛型更难。
我宁愿将步骤分解成非常小的部分，并尽可能避免链接。有了这个，你基本上需要为每一步添加特征界限。一件好事是为非常复杂的特征边界使用/创建类型别名。 Diesel provides a number of these，你可以自己做特殊用途。
当我查看生成的错误消息时，我主要查看被调用的函数/方法描述的类型边界。
逐点：
.find来自FindDsl。delete需要IntoUpdateTarget。delete的结果类型是DeleteStatement，用T::Table和T::WhereClause参数化。这是自定义类型别名DeleteFindStatement。.execute来自ExecuteDsl。use diesel::{
associations::HasTable,
helper_types::Find,
query_builder::{DeleteStatement, IntoUpdateTarget},
query_dsl::methods::ExecuteDsl,
};
type DeleteFindStatement<F> =
DeleteStatement<<F as HasTable>::Table, <F as IntoUpdateTarget>::WhereClause>;
impl DB {
pub fn remove_row<Tbl, Pk>(&self, table: Tbl, pk: Pk) -> Result<(), Box<Error>>
where
Tbl: FindDsl<Pk>,
Find<Tbl, Pk>: IntoUpdateTarget,
DeleteFindStatement<Find<Tbl, Pk>>: ExecuteDsl<SqliteConnection>,
{
let find = table.find(pk);
let delete = diesel::delete(find);
delete.execute(&self.conn)?;
Ok(())
}
}
您将需要自己尝试基于filter的版本，因为您没有提供足够的代码来告诉id应该是什么;如您的错误消息中所示。
也可以看看：
即使在所有情况下都是
&str，使主键通用是否有好处？
对我来说，使用泛型类型比插入一堆通用生命周期参数更容易。