Rust Diesel定义PostgreSQL二进制字段遇Expression trait报错求解决

问题场景 #

使用如下配置的Rust Diesel：

diesel = { version = "2.1.0", features = [ "postgres", "64-column-tables", "chrono", "serde_json", ] }

定义插入模型时编写了以下代码：

#![allow(unused)]
#![allow(clippy::all)]
use std::ffi::OsString;
use actix_multipart::form::tempfile::TempFile;
use diesel::sql_types::Bytea;
use rust_wheel::common::util::time_util::get_current_millisecond;
use rust_wheel::model::user::login_user_info::LoginUserInfo;
use serde::Serialize;
use serde::Deserialize;
use uuid::Uuid;
use crate::model::diesel::tex::custom_tex_models::TexTemplate;
use crate::model::diesel::tex::tex_schema::*;
use crate::model::request::file::add::file_add_req::TexFileAddReq;
use crate::model::request::file::add::file_add_ver_req::TexFileVerAddReq;

#[derive(Insertable,Queryable,QueryableByName,Debug,Serialize,Deserialize,Default,Clone)]
#[diesel(table_name = tex_file_version)]
pub struct TexFileVersionAdd {
    pub name: String,
    pub created_time: i64,
    pub updated_time: i64,
    pub user_id: i64,
    pub project_id: String,
    pub file_id: String,
    pub content: String,
    pub snapshot: Bytea
}

编译时触发错误：

~/source/reddwarf/backend/texhub-server on  main! ⌚ 12:29:48
$ cargo b                                                                                                                 ‹ruby-2.7.2›
   Compiling texhub-server v0.1.0 (/Users/dolphin/source/reddwarf/backend/texhub-server)
error[E0277]: the trait bound `diesel::sql_types::Binary: diesel::Expression` is not satisfied
   --> src/model/diesel/tex/custom_tex_models.rs:107:9
    |
107 |     pub snapshot: Bytea,
    |         ^^^^^^^^ the trait `diesel::Expression` is not implemented for `diesel::sql_types::Binary`
    |
    = help: the following other types implement trait `diesel::Expression`:
              &'a T
              (T0, T1)
              (T0, T1, T2)
              (T0, T1, T2, T3)
              (T0, T1, T2, T3, T4)
              (T0, T1, T2, T3, T4, T5)
              (T0, T1, T2, T3, T4, T5, T6)
              (T0, T1, T2, T3, T4, T5, T6, T7)
            and 240 others
    = note: required for `diesel::sql_types::Binary` to implement `AsExpression<diesel::sql_types::Binary>`

解决方法 #

错误核心是直接用Diesel的SQL类型Bytea作为结构体字段类型，而Diesel要求结构体字段使用对应Rust类型，SQL类型仅用于schema定义或类型注解。

针对PostgreSQL的bytea类型，Diesel映射的标准Rust类型是Vec<u8>，修改步骤如下：

1. 将字段类型从Bytea改为Vec<u8>
2. 若使用QueryableByName特性，需给字段添加#[diesel(sql_type = Bytea)]注解，显式绑定对应SQL类型

修改后的代码：

#![allow(unused)]
#![allow(clippy::all)]
use std::ffi::OsString;
use actix_multipart::form::tempfile::TempFile;
use diesel::sql_types::Bytea;
use rust_wheel::common::util::time_util::get_current_millisecond;
use rust_wheel::model::user::login_user_info::LoginUserInfo;
use serde::Serialize;
use serde::Deserialize;
use uuid::Uuid;
use crate::model::diesel::tex::custom_tex_models::TexTemplate;
use crate::model::diesel::tex::tex_schema::*;
use crate::model::request::file::add::file_add_req::TexFileAddReq;
use crate::model::request::file::add::file_add_ver_req::TexFileVerAddReq;

#[derive(Insertable,Queryable,QueryableByName,Debug,Serialize,Deserialize,Default,Clone)]
#[diesel(table_name = tex_file_version)]
pub struct TexFileVersionAdd {
    pub name: String,
    pub created_time: i64,
    pub updated_time: i64,
    pub user_id: i64,
    pub project_id: String,
    pub file_id: String,
    pub content: String,
    #[diesel(sql_type = Bytea)]
    pub snapshot: Vec<u8>
}

关键说明 #

• Bytea是Diesel定义的SQL类型标识，仅用于声明字段对应的数据库类型，不能直接作为Rust结构体字段类型
• Vec<u8>是Diesel为PostgreSQLbytea类型提供的默认Rust映射类型，同时兼容Serialize/Deserialize特性
• 若通过diesel print-schema生成数据库schema，生成的代码中bytea字段会自动对应Vec<u8>，结构体字段只需保持一致即可

内容的提问来源于stack exchange，提问作者Dolphin