锈柴油orm查询

Question

我是生锈和柴油机的新手。我正在尝试对我的查询执行以下操作：

• 计数
• 选择
• 订购
• 限制

但我遇到了错误。
我正在使用 postgres 数据库。

我在评论中的查询上方添加了确切的错误。
这是我的代码：

schema.rs

table! {
    employee (employee_id) {
        employee_id -> Int4,
        name -> Nullable<Text>,
        age -> Nullable<Int4>,
        address -> Nullable<Text>,
        email -> Nullable<Text>,
        dept_id -> Int4,
        salary -> Nullable<Numeric>,
        created_on -> Nullable<Timestamp>,
        created_by -> Nullable<Text>,
        modified_on -> Nullable<Timestamp>,
        modified_by -> Nullable<Text>,
        is_active -> Nullable<Bool>,
    }
}

models.rs

#![allow(unused)]
#![allow(clippy::all)]
use super::schema::employee;

use bigdecimal::BigDecimal;
use chrono::NaiveDateTime;

#[derive(Queryable, Debug, Identifiable)]
#[table_name = "employee"]
#[primary_key(employee_id)]
pub struct Employee {
    pub employee_id: i32,
    pub name: Option<String>,
    pub age: Option<i32>,
    pub address: Option<String>,
    pub email: Option<String>,
    pub dept_id: i32,
    pub salary: Option<BigDecimal>,
    pub created_on: Option<NaiveDateTime>,
    pub created_by: Option<String>,
    pub modified_on: Option<NaiveDateTime>,
    pub modified_by: Option<String>,
    pub is_active: Option<bool>,
}

cargo.toml

[dependencies]
diesel = { version = "1.4.5", features = ["postgres","chrono","numeric"] }
dotenv = "0.15.0"
chrono = { version = "0.4.19" , features = ["serde"] }
bigdecimal = { version = "0.1.0" }

ma​​in.rs

#[macro_use]
extern crate diesel;
extern crate bigdecimal;
extern crate chrono;
extern crate dotenv;

use crate::models::Employee;
use crate::models::Players;
use crate::schema::employee::dsl::*;

use diesel::{pg::PgConnection, prelude::*};
use dotenv::dotenv;
use std::env;

mod models;
mod schema;

fn main() {
    dotenv().ok();
    let data_url: String = env::var("DATABASE_URL").expect("DATABASE_URL must be set");
    let connection: PgConnection =
        PgConnection::establish(&data_url).expect(&format!("Error connect to {}", data_url));

    //get all employees name
    //This is working fine
    let _employee: Vec<Employee> = employee
        .load::<Employee>(&connection)
        .expect("Error loading department");

    for emp in _employee {
        println!("{}", emp.name.unwrap_or_default());
    }


    //----------------------------------------------
    //get employees count
    /*
    Error: error[E0282]: type annotations needed
           ^^^^^^^^^^^^^^^ consider giving `total_employees` a type
    */
    let total_employees = employee.count().get_result(&connection).expect("Error");
    println!("{}", total_employees);


    //-----------------------------------------------
     //get all names
    /*
        Error: error[E0277]: the trait bound `*const str: FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not satisfied
         ^^^^ the trait `FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not implemented for `*const str`
    */
    let all_names = employee.select(name).load::<String>(&connection)?;
    println!("{}", all_names);


    //----------------------------------------------
    //order name
    /*
    Error: error[E0277]: the trait bound `*const str: FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not          satisfied
    ^^^^ the trait `FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not implemented for `*const str`
    */
    let ordered_names = employee
        .select(name)
        .order(name.desc())
        .load::<String>(&connection)?;
    println!("{}", ordered_names);


   //------------------------------------------------
   /*
    Error: error[E0277]: the trait bound `*const str: FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not     satisfied
    ^^^^ the trait `FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not implemented for `*const str`
     */
    let limited = employee
        .select(name)
        .order(employee_id)
        .limit(1)
        .load::<String>(&connection)?;
    println!("{}", limited);
}

我错过了什么吗？ 有人可以纠正我吗？
谢谢！

Answer 1

首先：您的代码缺少实际重现那里描述的问题的关键信息。在不知道您的底层数据库架构的情况下，只能猜测它的样子。对于我的回答，我将假设以下架构：

table! {
    employee(employee_id) {
        employee_id -> Integer,
        name -> Nullable<Text>,
        age -> Nullable<Integer>,
        address -> Nullable<Text>,
        email -> Nullable<Text>,
        dept_id -> Integer,
        salary -> Nullable<Numeric>,
        created_on -> Nullable<Timestamp>,
        created_by -> Nullable<Text>,
        modified_on -> Nullable<Timestamp>,
        modified_by -> Nullable<Text>,
        is_active -> Nullable<Bool>,
    }
}

现在作为一般规则：Diesel 将此定义用作事实来源，并且仅支持某些类型映射。如果您遇到编译错误，这可能意味着以下情况之一是错误的：

• 您已尝试将查询结果映射到包含某些返回列的不兼容字段的结构
• 您尝试将查询结果映射到包含比查询返回的更多或更少字段的结构

回答您的具体错误信息：

    //----------------------------------------------
    //get employees count
    /*
    Error: error[E0282]: type annotations needed
           ^^^^^^^^^^^^^^^ consider giving `total_employees` a type
    */
    let total_employees = employee.count().get_result(&connection).expect("Error");
    println!("{}", total_employees);

Rustc 需要知道total_employees 的类型，因为get_result 返回一个泛型类型，而println 使用一个泛型类型。 Rustc 需要确切地知道应该在那里使用哪种类型。现在柴油文档在正确的返回类型上有点稀疏，但Daniel Porteous 中的错误消息表明此查询返回一个BigInt，它与i64 兼容，如here 所述。这意味着这个查询可以工作：

    let total_employees: i64 = employee.count().get_result(&connection).expect("Error");

接下来的三个查询基本上无法编译并出现相同的错误消息：

    //-----------------------------------------------
     //get all names
    /*
        Error: error[E0277]: the trait bound `*const str: FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not satisfied
         ^^^^ the trait `FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not implemented for `*const str`
    */
    let all_names = employee.select(name).load::<String>(&connection)?;
    println!("{}", all_names);


    //----------------------------------------------
    //order name
    /*
    Error: error[E0277]: the trait bound `*const str: FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not          satisfied
    ^^^^ the trait `FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not implemented for `*const str`
    */
    let ordered_names = employee
        .select(name)
        .order(name.desc())
        .load::<String>(&connection)?;
    println!("{}", ordered_names);


   //------------------------------------------------
   /*
    Error: error[E0277]: the trait bound `*const str: FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not     satisfied
    ^^^^ the trait `FromSql<diesel::sql_types::Nullable<diesel::sql_types::Text>, _>` is not implemented for `*const str`
     */
    let limited = employee
        .select(name)
        .order(employee_id)
        .limit(1)
        .load::<String>(&connection)?;
    println!("{}", limited);

现在，此错误消息表明您尝试将查询返回的字段映射到 rust 端的不兼容类型。在这种情况下，不支持将不是NOT NULL 的Text 字段映射到String，因为在这种情况下柴油如何表示NULL 值。 documentation states 需要将可空类型包装在 Option 生锈方面。这意味着如果您在所有情况下将返回类型更改为Option<String>，一切都会成功编译。

Answer 2

let total_employees = employee.count().get_result(&connection).expect("Error");

对于这个，尝试注释total_employees，如下所示：

let total_employees: u64 = employee.count().get_result(&connection).expect("Error");

这将帮助您继续。即使函数没有返回这样的类型，错误现在至少会告诉你它是什么类型。