Performing a Right Join in Diesel

Question

I have a function that takes two optional filtering arguments, and filters the data in a table if these arguments are provided. To do this, I created a boxed query. I would like to create a right join with this boxed query. Diesel's current documentation doesn't mention right join, but it seems to prefer the belonging_to method. I have reduced my code to a single example:

#[macro_use] extern crate diesel;

use diesel::prelude::*;

table! {
    groups (id) {
        id -> Int4,
        name -> Text,
    }
}

table! {
    user_groups (id) {
        id -> Int4,
        user_id -> Int4,
        group_id -> Int4,
    }
}

allow_tables_to_appear_in_same_query!(groups, user_groups);

#[derive(Debug, Queryable)]
struct Group {
    id: i32,
    name: String,
}

#[derive(Debug, Queryable, Associations)]
#[belongs_to(Group)]
struct UserGroup {
    id: i32,
    user_id: i32,
    group_id: i32,
}

fn filter(
    name: Option<&str>,
    user_id: Option<i32>,
    conn: &diesel::PgConnection,
) -> Result<Vec<(Group, Vec<UserGroup>)>, Box<dyn std::error::Error>> {
    let mut query = groups::table
        .right_join(user_groups::table.on(groups::id.eq(user_groups::group_id))) // this method does not exist
        .into_boxed();
    if let Some(name) = name {
        query = query.filter(groups::name.eq(name));
    }
    if let Some(user_id) = user_id {
        query = query.filter(user_groups::user_id.contains(user_id)); // so this is just a guess
    }
    Ok(query.get_results(conn)?)
}

fn main() {
    let url = "postgres://question_usr:question_pwd:localhost:5432/question_db";
    let conn = diesel::PgConnection::establish(url).unwrap();
    let _ = filter(Some("groupname"), Some(4), &conn).unwrap();
}

The intent is that if the argument user_id is specified, only rows of Groups are returned which have at least one UserGroup such that user_group.user_id == user_id. How do I run this query? Do I need to use the belonging_to function somehow?

Answer 1

A right join is in every case equivalent to a left join with inverted table order. (See here for example). Therefore Diesel only provides functions to construct a LEFT JOIN.

Your example then looks like this:

fn filter(
    name: Option<&str>,
    user_id: Option<i32>,
    conn: &diesel::PgConnection,
) -> Result<Vec<(UserGroup, Option<Group>)>, Box<dyn std::error::Error>> {
    let mut query = user_groups::table
        .left_join(groups::table.on(groups::id.eq(user_groups::group_id))) /
        .into_boxed();
    if let Some(name) = name {
        query = query.filter(groups::name.eq(name));
    }
    if let Some(user_id) = user_id {
        query = query.filter(user_groups::user_id.eq(user_id)); 
    }
    Ok(query.get_results(conn)?)
}

Please note that this returns a Vec<(UserGroup, Option<Group>)> by default. If you want something different you need to write a custom select clause specifying the required result shape.