用柴油机r2d2实现数据库模块中的默认特征API？

Question

我对生锈/柴油很陌生。尝试用默认特性API (避免重复代码工作)，通过机箱柴油和r2d2来实现数据库模块。

这是model.rs

use chrono::NaiveDate;
use diesel;
use diesel::Queryable;

#[derive(Queryable)]
pub struct NetWorthModel {
    pub fund_code: String,
    pub date: NaiveDate,
    pub create_time: i64,
    pub update_time: i64,
    pub payload: String,
}

这是database.rs

use diesel::connection::Connection;
use diesel::mysql::MysqlConnection;
use diesel::prelude::*;
use diesel::r2d2::{ConnectionManager, Pool};
use diesel::sqlite::SqliteConnection;
use once_cell::sync::Lazy;
use r2d2::PooledConnection;
use std::collections::HashMap;
use std::sync::{Arc, Mutex};
use chrono::prelude::NaiveDate;

static MYSQL_POOL_MAP: Lazy<Mutex<HashMap<String, Pool<ConnectionManager<MysqlConnection>>>>> =
    Lazy::new(|| Mutex::new(HashMap::new()));

static SQLITE_POOL_MAP: Lazy<Mutex<HashMap<String, Pool<ConnectionManager<SqliteConnection>>>>> =
    Lazy::new(|| Mutex::new(HashMap::new()));

pub struct MysqlDatabase {
    pool: Pool<ConnectionManager<MysqlConnection>>,
}

pub struct SqliteDatabase {
    pool: Pool<ConnectionManager<SqliteConnection>>,
}

pub trait Database<Conn> {
  fn connection(&self) -> Conn; // MysqlDatabase and SqliteDatabase will ipmlement this api

  fn paged_query(&self, fund_code: &str, start_date: NaiveDate, end_date: NaiveDate) -> Vec<NetWorthModel> {
    use super::schema::tb_net_worth::dsl;
    dsl::tb_net_worth
         .filter(dsl::fund_code.eq(fund_code))
         .filter(dsl::date.ge(start_date))
         .filter(dsl::date.lt(end_date))
         .load::<NetWorthModel>(&self.connection()) // here we use the abstract API
         .unwrap()
  }
}

impl Database<PooledConnection<ConnectionManager<MysqlConnection>>> for MysqlDatabase {
    fn connection(&self) -> PooledConnection<ConnectionManager<MysqlConnection>> {
        self.pool.get().unwrap()
    }
}

impl Database<PooledConnection<ConnectionManager<SqliteConnection>>> for SqliteDatabase {
    fn connection(&self) -> PooledConnection<ConnectionManager<SqliteConnection>> {
        self.pool.get().unwrap()
    }
}

Database特性将实现所有的API，这些API应该同时适用于mysql和sqlite。但是，下面是编译错误：

error[E0277]: the trait bound `Conn: Connection` is not satisfied
   --> src/quant/common/persistence/database.rs:221:45
    |
221 |                 .load::<NetWorthQueryModel>(&self.connection())
    |                  ----                       ^^^^^^^^^^^^^^^^^^ the trait `Connection` is not implemented for `Conn`
    |                  |
    |                  required by a bound introduced by this call
    |
    = note: required because of the requirements on the impl of `LoadQuery<Conn, NetWorthQueryModel>` for `diesel::query_builder::SelectStatement<table, diesel::query_builder::select_clause::DefaultSelectClause, diesel::query_builder::distinct_clause::NoDistinctClause, diesel::query_builder::where_clause::WhereClause<And<And<diesel::expression::operators::Eq<columns::fund_code, diesel::expression::bound::Bound<diesel::sql_types::Text, &str>>, GtEq<columns::date, diesel::expression::bound::Bound<diesel::sql_types::Date, NaiveDate>>>, Lt<columns::date, diesel::expression::bound::Bound<diesel::sql_types::Date, NaiveDate>>>>, diesel::query_builder::order_clause::OrderClause<Asc<columns::date>>, diesel::query_builder::limit_clause::LimitClause<diesel::expression::bound::Bound<diesel::sql_types::BigInt, i64>>, diesel::query_builder::offset_clause::OffsetClause<diesel::expression::bound::Bound<diesel::sql_types::BigInt, i64>>>`
help: consider restricting type parameter `Conn`
    |
147 | pub trait Database<Conn: diesel::Connection> {
    |                        ++++++++++++++++++++

按照编译器的建议，我添加了pub trait Database<Conn: diesel::Connection>，编译器给出了另一个错误：

error[E0277]: the trait bound `NaiveDate: FromSql<diesel::sql_types::Date, <Conn as Connection>::Backend>` is not satisfied
   --> src/quant/common/persistence/database.rs:221:18
    |
221 |                 .load::<NetWorthQueryModel>(&self.connection())
    |                  ^^^^ the trait `FromSql<diesel::sql_types::Date, <Conn as Connection>::Backend>` is not implemented for `NaiveDate`
    |
    = note: required because of the requirements on the impl of `diesel::Queryable<diesel::sql_types::Date, <Conn as Connection>::Backend>` for `NaiveDate`
    = note: 2 redundant requirements hidden
    = note: required because of the requirements on the impl of `diesel::Queryable<(diesel::sql_types::Text, diesel::sql_types::Date, diesel::sql_types::BigInt, diesel::sql_types::BigInt, diesel::sql_types::Text), <Conn as Connection>::Backend>` for `NetWorthQueryModel`
    = note: required because of the requirements on the impl of `LoadQuery<Conn, NetWorthQueryModel>` for `diesel::query_builder::SelectStatement<table, diesel::query_builder::select_clause::DefaultSelectClause, diesel::query_builder::distinct_clause::NoDistinctClause, diesel::query_builder::where_clause::WhereClause<And<And<diesel::expression::operators::Eq<columns::fund_code, diesel::expression::bound::Bound<diesel::sql_types::Text, &str>>, GtEq<columns::date, diesel::expression::bound::Bound<diesel::sql_types::Date, NaiveDate>>>, Lt<columns::date, diesel::expression::bound::Bound<diesel::sql_types::Date, NaiveDate>>>>, diesel::query_builder::order_clause::OrderClause<Asc<columns::date>>, diesel::query_builder::limit_clause::LimitClause<diesel::expression::bound::Bound<diesel::sql_types::BigInt, i64>>, diesel::query_builder::offset_clause::OffsetClause<diesel::expression::bound::Bound<diesel::sql_types::BigInt, i64>>>`
help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
    |
147 | pub trait Database<Conn: Connection> where NaiveDate: FromSql<diesel::sql_types::Date, <Conn as Connection>::Backend> {
    |                                      ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

看起来锈菌无法检测到Backend of Connection。但我不知道该怎么解决这个问题。

请帮我解决这个问题。

Answer 1

解决这一问题的正确方法是遵循编译器的错误消息，并引入必要的特征界限。没有他们这是行不通的。作为一般建议:尝试使用required because of给出的最后一个边界，因为这样可以避免许多其他界限。

一般来说:如果你是柴油新手，而且生锈了，那么尝试编写试图抽象柴油的通用代码可能是不可取的。这需要预先了解生锈特性系统+柴油API。