CODEWITHSUNDEEP
rustrust-diesel
user51
user51

Reputation: 10243

The trait diesel::Connection is not implemented for &diesel::MysqlConnection

I'm trying to learn Diesel following this tutorial. This is the demo program I've created using Diesel:

#![recursion_limit = "128"]

#[macro_use]
extern crate diesel;
#[macro_use]
extern crate diesel_infer_schema;

extern crate dotenv;
use diesel::mysql::MysqlConnection;
use diesel::prelude::*;
use dotenv::dotenv;
use std::env;

pub fn establish_connection() -> MysqlConnection {
    dotenv().ok();
    let db_url: String = String::from(env::var("DB_URL").expect("DB_URL must be set"));
    let db_connection =
        MysqlConnection::establish(&db_url).expect(&format!("Error connecting to {}", &db_url));

    return db_connection;
}

pub mod schema {
    infer_schema!("dotenv:DB_URL");
}

use schema::*;

table! {
    tag {
        id -> SmallInt,
        tag_name -> Varchar,
    }
}

#[derive(Queryable, Insertable)]
#[table_name = "tag"]
pub struct Tag {
    pub id: i16,
    pub tag_name: String,
}

fn read_and_output(db_connection: &MysqlConnection) {
    let results = tag::table.load::<Tag>(&db_connection).expect("problem");
    println!("Returned results: {}", results.len());
    for r in results {
        println!("{} {}", r.id, r.tag_name);
    }
}

pub fn insert_tag(db_connection: &MysqlConnection, tag_id_val: i16, tag_name_val: String) {
    let new_tag = Tag {
        id: tag_id_val,
        tag_name: tag_name_val,
    };

    diesel::insert_into(tag::table)
        .values(&new_tag)
        .execute(db_connection)
        .expect("Error inserting");
}

fn main() {
    println!("Hello, world!");
    let db_connection = establish_connection();

    // 1. query data from the table
    //read_and_output(&db_connection);

    // 2. insert new data into the table
    let tag_id: i16 = 778;
    let tag_name: String = String::from("educational");
    insert_tag(&db_connection, tag_id, tag_name);
    //read_and_output(&db_connection);
}

The complete code is available in my GitHub repository.

When I compile the program using cargo build, I get this error:

error[E0277]: the trait bound `&diesel::MysqlConnection: diesel::Connection` is not satisfied
--> src/main.rs:45:30
   |
45 |     let results = tag::table.load::<Tag>(&db_connection)
   |                              ^^^^ the trait `diesel::Connection` is not implemented for `&diesel::MysqlConnection`
   |
= help: the following implementations were found:
            <diesel::MysqlConnection as diesel::Connection>
= note: required because of the requirements on the impl of `diesel::query_dsl::LoadQuery<&diesel::MysqlConnection, Tag>` for `tag::table`

When I comment out the read_and_output function and its invocation, the code compiles and the records are inserted into database.

I'm not sure how to resolve this error; it looks like I need to implement some trait but not sure how can I do that.

Upvotes: 3

Views: 2115

Answers (1)

twhitehead
twhitehead

Reputation: 81

In your code db_connection is already a reference to a MysqlConnection, i.e. of type &MysqlConnection, so just pass db_connection and not &db_connection like so 

let results = tag::table.load::<Tag>(db_connection);

Upvotes: 1

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