pickfire / diesel-multi-join-play

Rust diesel multiple joins experiment

Diesel multi-join experiment

Next: https://github.com/pickfire/diesel-update-from

After getting footguns in sequelize and gorm, just want to try out diesel and state of diesel.

So far, a bit hard to get started as error message are a bit confusing, for example you get "failed to resolve: use of undeclared crate or module new_users" instead of missing #[table_name = "users"] if you removed that line from pub struct NewUser.

Sometimes compile type errors are also a bit overwhelming. Especially when you got into a case not supported by diesel, like keeping original type when joining. I wish these are mentioned in the docs too, like what was not supported but beginners might try it out when using it.

error[E0277]: the trait bound `(i32, String): Queryable<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Integer, diesel::sql_types::Integer), Sqlite>` is not satisfied
  --> src/main.rs:17:50
   |
17 |     let data: Vec<(Comment, Post, User)> = query.load(&conn).unwrap();
   |                                                  ^^^^ the trait `Queryable<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Integer, diesel::sql_types::Integer), Sqlite>` is not implemented for `(i32, String)`
   |
   = help: the following implementations were found:
             <(A, B) as Queryable<(SA, SB), __DB>>
   = note: required because of the requirements on the impl of `Queryable<(diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Integer, diesel::sql_types::Integer), Sqlite>` for `hello::models::Comment`
   = note: required because of the requirements on the impl of `Queryable<((diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Integer, diesel::sql_types::Integer), (diesel::sql_types::Integer, diesel::sql_types::Text, diesel::sql_types::Nullable<diesel::sql_types::Text>, diesel::sql_types::Integer), (diesel::sql_types::Integer, diesel::sql_types::Text)), Sqlite>` for `(hello::models::Comment, hello::models::Post, hello::models::User)`
   = note: required because of the requirements on the impl of `LoadQuery<SqliteConnection, (hello::models::Comment, hello::models::Post, hello::models::User)>` for `SelectStatement<JoinOn<diesel::query_source::joins::Join<JoinOn<diesel::query_source::joins::Join<hello::schema::comments::table, hello::schema::posts::table, Inner>, diesel::expression::operators::Eq<diesel::expression::nullable::Nullable<post_id>, diesel::expression::nullable::Nullable<hello::schema::posts::id>>>, hello::schema::users::table, Inner>, diesel::expression::operators::Eq<diesel::expression::nullable::Nullable<hello::schema::comments::user_id>, diesel::expression::nullable::Nullable<hello::schema::users::id>>>>`

Docs examples are also hard to get by at first. For example, how to establish a new connection, I looked into SqliteConnection but did not noticed that it is covered within Connection trait implemented by SqliteConnection, maybe diesel::connection could mention it?

But until then, I never experienced any weird behaviors like incorrect behavior, still getting the experience of "when it compiles, it works" but note that it may take some effort for that, once someone did a few tries in diesel I think it will be easier.

I think the type-safe effort is still worth it, rather than figure out why the type magic does not work, now just need to get it compile and it works.

The example I tested out here is to have 3 struct, and have multiple inner joins. I use sqlite for this for easy testing. Even though it works but I wish it have better ergonomics when dealing with joins. Like being able to just extract original struct instead of each field.

+---------+     +---------+     +---------+
| User    |<-+  | Post    |     | Comment |
+---------+  |  +---------+     +---------+
| id      |  |  | id      |     | id      |
| name    |  |  | title   |     | body    |
|         |  |  | body    |<----+ post_id |
|         |  |  | user_id +--+--+ user_id |
+---------+  |  +---------+  |  +---------+
             +---------------+

I was just looking to get all comments commented by user John. The query,

let query = comments
    .inner_join(posts)
    .inner_join(users)
    .select((comments::columns::body, title, name))
    .filter(name.eq("John"));

Which results in the SQL query,

SELECT
  `comments`.`body`,
  `posts`.`title`,
  `users`.`name`
 FROM ((`comments`
INNER JOIN `posts`
   ON `comments`.`post_id` = `posts`.`id`)
INNER JOIN `users`
   ON `comments`.`user_id` = `users`.`id`)
WHERE `users`.`name` = ? -- binds: ["John"]

Get started

Rust, diesel_cli (with sqlite feature) is required.

$ diesel migration run
$ cargo run --bin init  # populate database
$ cargo run --bin hello  # multi join query