rust - 在用户不使用N + 1查询的情况下登录时将数据添加到Diesel结果中

Question

我有一个电影数据库，用户可以使用以下模型将电影标记为已观看和收藏的电影:

#[derive(Serialize, Deserialize, Identifiable, Queryable, Associations)]
#[table_name = "movies"]
struct Movie {
    id: 32
}

#[derive(Serialize, Deserialize, Identifiable, Queryable, Associations)]
#[table_name = "users"]
struct User {
    id: 32
}

#[derive(Serialize, Deserialize, Identifiable, Queryable, Associations)]
#[table_name = "favorite_movies"]
struct FavoriteMovie {
    movie_id: i32,
    user_id: i32,
}

#[derive(Serialize, Deserialize, Identifiable, Queryable, Associations)]
#[table_name = "watched_movies"]
struct WatchedMovie {
    movie_id: i32,
    user_id: i32,
    date: DateTime,
}

如果要登录，同时又要避免N + 1查询的问题，该如何处理要列出数据库中最近添加的100部电影并包括每部电影的用户数据的情况？

我在想可以为movies表添加另一个结构，其中还包括可选的用户数据，但是我不确定如何将其与Diesel ORM一起使用。

struct UserData {
   favorite: bool,
   watched: Option<DateTime>,
}

struct UserMovie {
   id: i32,
   user_data: Option<UserData>,
}

Answer 1

使用 left_join :

struct UserMovie {
    movie: Movie,
    favorite_movie: Option<FavoriteMovie>,
    watched_movie: Option<WatchedMovie>,
}

let conn = get_db_connection();
let user_movies: Vec<UserMovie> = movies::table
    .left_join(
        favorite_movies::table.on(favorite_movies::movie_id
            .eq(movies::id)
            .and(favorite_movies::user_id.eq(user_id))),
    )
    .left_join(
        watched_movies::table.on(watched_movies::movie_id
            .eq(movies::id)
            .and(watched_movies::user_id.eq(user_id))),
    )
    .load(conn)?
    .into_iter()
    .map(
        |(movie, favorite_movie, watched_movie): (Movie, Option<FavoriteMovie>, Option<WatchedMovie>)| UserMovie {
            movie,
            favorite_movie,
            watched_movie,
        }
    )
    .collect()