Eager loading of relations

Question

I have three models with these relations:

Artist 1<->n Song 1<->n Play

I want to load Plays and eagerly load its Songs and its Artists.

I did this in PHP framework Eloquent easily. And it takes only 3 DB requests (one for each model).

Is this some how possible with diesel?

I think the Play model should look something like this:

#[derive(Queryable, Serialize, Deserialize)]
pub struct Play {
    pub id: u64,
    pub song_id: u64,
    pub song: Song, // <-- ????
    pub date: NaiveDateTime,
    pub station_id: u64,
}

And loading something like this:

// loads one model only at the moment
// it should load the related models Song and Artist too
let items = plays.filter(date.between(date_from, date_to)).load(&*conn)?;

The resulting struct should be JSON serialized to be used by the REST API. But I can't find a way to get the needed struct in one simple and efficient way.

Answer 1

First of all: If you post a question about diesel online, please always include the relevant part of your schema, otherwise others need to guess how it looks like. I will assume the following schema for your question:

table! {
    plays (id)
       id -> BigInt,
       song_id -> BigInt,
       date -> Timestamp,
       station_id -> BigInt,
    }

}

table! {
    songs(id) {
        id -> BigInt,
        name -> Text,
    }
}

joinable!(plays -> songs (song_id));

Now there is some issue with your Play struct. u64 is not a type supported by diesel. Checkout this documentation about which types are compatible with a BigInt SQL type. That means I will assume the following structs implementing Queryable:

#[derive(Queryable, Serialize, Deserialize)]
pub struct Play {
    pub id: i64,
//    pub song_id: u64, leave this out as it is a duplicate with `song` below
    pub song: Song,
    pub date: NaiveDateTime,
    pub station_id: i64,
}

#[derive(Queryable, Serialize, Deserialize)]
pub struct Song {
    pub id: i64,
    pub name: String,
}

Now to come to your core question: At first, diesel does not have the concept of a single model like known from other ORM's. If you have ever used such a thing, don't try to apply that knowledge to diesel, it won't work. There are a set of traits describing a capability of each struct. As we are talking about querying here I will concentrate on Queryable. Queryable is designed to map the result of a query (which could include zero, one or more tables) to a rust data structure. By default (by using the derive) it does this by mapping fields structural, that means your select clause needs to match the structure deriving Queryable. Now back to your concrete problem. It means that we must construct a query that returns (i64, (i64, String), NaiveDateTime, i64) (or better the fields with the corresponding SQL types). For this case this is as easy as using a single join:

plays::table.inner_join(songs::table)
   .select((plays::id, songs::all_columns, plays::date, plays::station_id))

The only important thing here for the mapping is the select clause. You can append/prepend whatever other QueryDsl method you like.