Rust 理解柴油机的特征界误差
我想写一个函数,在数据库连接参数为泛型的数据库中插入一个类型,这样它就可以在多个后端上工作 我使用以下函数使用通用连接插入对象:Rust 理解柴油机的特征界误差,rust,rust-diesel,Rust,Rust Diesel,我想写一个函数,在数据库连接参数为泛型的数据库中插入一个类型,这样它就可以在多个后端上工作 我使用以下函数使用通用连接插入对象: pub fn create_label<C>(connection: &C, label: &model::Label) where C: Connection, C::Backend: diesel::backend::Backend, C::Backend: diesel::backend::SupportsDe
pub fn create_label<C>(connection: &C, label: &model::Label)
where
C: Connection,
C::Backend: diesel::backend::Backend,
C::Backend: diesel::backend::SupportsDefaultKeyword,
{
diesel::insert(&label)
.into(schema::label::table)
.execute(connection);
}
为什么泛型函数需要SupportsDefaultKeyword
约束,而使用SqliteConnection
的函数则不需要
下面是一个例子来说明这个问题。根据注释,main.rs
的第60行不会编译上面的错误,而第61行编译:
#[macro_use]
extern crate diesel;
#[macro_use]
extern crate diesel_codegen;
mod schema {
table! {
labels {
id -> Integer,
name -> VarChar,
}
}
}
mod model {
use schema::labels;
#[derive(Debug, Identifiable, Insertable)]
#[table_name = "labels"]
pub struct Label {
pub id: i32,
pub name: String,
}
}
use diesel::ExecuteDsl;
use diesel::Connection;
use diesel::prelude::*;
use diesel::sqlite::SqliteConnection;
pub fn create_label<C>(connection: &C, label: &model::Label)
where
C: Connection,
C::Backend: diesel::backend::Backend,
C::Backend: diesel::backend::SupportsDefaultKeyword,
{
diesel::insert(label)
.into(schema::labels::table)
.execute(connection)
.expect("nope");
}
pub fn create_label_sqlite(connection: &SqliteConnection, label: &model::Label) {
diesel::insert(label)
.into(schema::labels::table)
.execute(connection)
.expect("nope");
}
pub fn establish_connection() -> SqliteConnection {
let url = "test.db";
SqliteConnection::establish(&url).expect(&format!("Error connecting to {}", url))
}
fn main() {
let label = model::Label {
id: 1,
name: String::from("test"),
};
let conn = establish_connection();
create_label(&conn, &label); /* Does not compile */
create_label_sqlite(&conn, &label); /*Compiles */
}
Diesel函数有多个具体实现。此处相关的两个方面是:
impl<'a, T, U, Op, Ret, Conn, DB> ExecuteDsl<Conn, DB> for BatchInsertStatement<T, &'a [U], Op, Ret>
where
Conn: Connection<Backend = DB>,
DB: Backend + SupportsDefaultKeyword,
InsertStatement<T, &'a [U], Op, Ret>: ExecuteDsl<Conn>,
impl
哪里
康涅狄格州:连接,
DB:Backend+SupportsDefaultKeyword,
BatchInsertStatement的InsertStatement ExecuteSL:ExecuteSL,
T:收到,
作品:复制,,
雷特:收到,
从这两个方面可以看出,SQLite的实现是特殊情况的。我对Diesel的细节了解不够,不知道为什么,但我猜SQLite缺少默认关键字
相反,您可以重新制定使用该特定语句的任何连接的要求:
use diesel::query_builder::insert_statement::InsertStatement;
pub fn create_label<C>(connection: &C, label: &model::Label)
where
C: Connection,
for<'a> InsertStatement<schema::labels::table, &'a model::Label>: ExecuteDsl<C>,
{
diesel::insert(label)
.into(schema::labels::table)
.execute(connection)
.expect("nope");
}
use diesel::query\u builder::insert\u语句::InsertStatement;
发布fn创建标签(连接:&C,标签:&model::标签)
哪里
C:连接,
for:executesl,
{
柴油机:插入(标签)
.into(架构::标签::表)
.执行(连接)
.预期(“不”);
}
impl<'a, T, U, Op, Ret, Conn, DB> ExecuteDsl<Conn, DB> for BatchInsertStatement<T, &'a [U], Op, Ret>
where
Conn: Connection<Backend = DB>,
DB: Backend + SupportsDefaultKeyword,
InsertStatement<T, &'a [U], Op, Ret>: ExecuteDsl<Conn>,
impl<'a, T, U, Op, Ret> ExecuteDsl<SqliteConnection> for BatchInsertStatement<T, &'a [U], Op, Ret>
where
InsertStatement<T, &'a U, Op, Ret>: ExecuteDsl<SqliteConnection>,
T: Copy,
Op: Copy,
Ret: Copy,
use diesel::query_builder::insert_statement::InsertStatement;
pub fn create_label<C>(connection: &C, label: &model::Label)
where
C: Connection,
for<'a> InsertStatement<schema::labels::table, &'a model::Label>: ExecuteDsl<C>,
{
diesel::insert(label)
.into(schema::labels::table)
.execute(connection)
.expect("nope");
}