how to do a count query when using rust diesel

Question

I am doing a count query using rust 1.59.0 diesel diesel = { version = "1.4.8", features = ["postgres","64-column-tables","chrono"] } following this document, this is my code looks like right now:

use diesel::{ExpressionMethods, QueryDsl, QueryResult, RunQueryDsl};
use diesel::dsl::count;
use rocket::serde::json::Json;
use rust_wheel::config::db::config;

pub fn channel_fav_count(req_channel_id: &i64) -> i32 {
    use crate::model::diesel::dolphin::dolphin_schema::article_favorites::dsl::*;
    let connection = config::establish_connection();
    let query = article_favorites
        .filter(channel_id.eq(req_channel_id));
    let query_result = query.select(count(id)).first(&connection);
    return query_result.unwrap_or(0);
}

when I compile the code, shows error like this:

error[E0277]: the trait bound `i32: FromSql<BigInt, Pg>` is not satisfied
    --> src/service/app/cruise/article/article_fav_service.rs:11:48
     |
11   |     let query_result = query.select(count(id)).first(&connection);
     |                                                ^^^^^ the trait `FromSql<BigInt, Pg>` is not implemented for `i32`

why did this happen? what should I do to fix this problem?

Answer 1

first thing you must know it's query count return i64 not i32, so you must convert i64 to i32 by your self

maybe like this

let query_result = query.select(count(id)).first(&connection).map(|x| x as i32);

also you can get count by filter query like below

users
    .filter(user_id.eq(_user_id))
    .count()
    .get_result(conn) // Result<i64, Error>

for your code it's like this

article_favorites
    .filter(channel_id.eq(req_channel_id))
    .count()
    .get_result(&connection)