Rust用Diesel查询PostgreSQL时出现E0277 trait绑定错误

问题场景 #

使用Rust构建GraphQL API时，通过Diesel查询PostgreSQL数据库，调用load::<User>方法编译时触发E0277错误，提示数据库返回类型与User结构体不兼容。

错误信息 #

error[E0277]: the trait bound `(Integer, diesel::sql_types::Text, diesel::sql_types::Text, diesel::sql_types::Text, diesel::sql_types::Text, diesel::sql_types::Text, diesel::sql_types::Nullable<diesel::sql_types::Bool>, diesel::sql_types::Nullable<diesel::sql_types::Bool>): load_dsl::private::CompatibleType<User, Pg>` is not satisfied
    --> src\resolvers\users_resolver.rs:11:23
     |
11   |         .load::<User>(connection)
     |          ----         ^^^^^^^^^^ the trait `load_dsl::private::CompatibleType<User, Pg>` is not implemented for `(Integer, diesel::sql_types::Text, diesel::sql_types::Text, diesel::sql_types::Text, diesel::sql_types::Text, diesel::sql_types::Text, diesel::sql_types::Nullable<diesel::sql_types::Bool>, diesel::sql_types::Nullable<diesel::sql_types::Bool>)`
     |          |
     |          required by a bound introduced by this call
     |
     = help: the following other types implement trait `load_dsl::private::CompatibleType<U, DB>`:
               (ST0, ST1)
               (ST0, ST1, ST2)
               (ST0, ST1, ST2, ST3)
               (ST0, ST1, ST2, ST3, ST4)
               (ST0, ST1, ST2, ST3, ST4, ST5)
               (ST0, ST1, ST2, ST3, ST4, ST5, ST6)
               (ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7)
               (ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7, ST8)
             and 24 others
     = note: required for `SelectStatement<FromClause<table>, DefaultSelectClause<FromClause<table>>, NoDistinctClause, ..., ..., ...>` to implement `LoadQuery<'_, _, User>`
     = note: the full type name has been written to 'D:\Documents\chat\server\target\debug\deps\server.long-type-9800167139566061747.txt'
note: required by a bound in `diesel::RunQueryDsl::load`
    --> C:\Users\cooke\.cargo\registry\src\github.com-1ecc6299db9ec823\diesel-2.0.4\src\query_dsl\mod.rs:1499:15
     |
1499 |         Self: LoadQuery<'query, Conn, U>,
     |               ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::load`

触发错误的查询函数 #

pub fn get_users() -> Vec<User> {
    use crate::schema::users::dsl::*;
    let connection = &mut establish_connection();
    let results = users
        .limit(5)
        .load::<User>(connection)
        .expect("Error loading posts");
    results
}

User模型定义 #

#[derive(Queryable)]
pub struct User {
    pub id: i32,
    pub first_name: String,
    pub last_name: String,
    pub email: String,
    pub password: String,
    pub phone: String,
    pub active: bool,
    pub superuser: bool,
}

解决方案 #

1. 匹配数据库字段可空性 #

错误提示显示数据库返回的active和superuser字段是Nullable<Bool>类型，但User模型中定义为非空bool，导致类型不兼容：

• 如果数据库中这两个字段确实允许为NULL，修改User模型对应字段为Option<bool>：

  #[derive(Queryable)]
  pub struct User {
      pub id: i32,
      pub first_name: String,
      pub last_name: String,
      pub email: String,
      pub password: String,
      pub phone: String,
      pub active: Option<bool>,
      pub superuser: Option<bool>,
  }
  

• 如果数据库中这两个字段不允许为NULL，检查src/schema.rs文件中对应的字段定义，确保是Bool而非Nullable<Bool>。若schema文件与数据库结构不一致，重新生成schema：

  diesel print-schema > src/schema.rs
  

2. 确认派生宏依赖 #

确保User结构体所在文件已正确导入Queryable trait：

use diesel::Queryable;

内容的提问来源于stack exchange，提问作者Isaac Cooke