.with and polymorphic relationships

[tintin10q]

Hi there,

I have now arrived at the point in my pinia orm journey that I need to start using polymorphic relationships. Things so far have been good. Besides the .with.

I have a schema like this:

class HumanSavedMovie extends Model {
  static entity = "HumanSavedEpisode";
  static primaryKey = ["human_id", "movie_id"];

  static fields() {
    return {
      movie_id: this.uid(),
      human_id: this.uid(),
      added_at: this.attr(utcsec()),
      human: this.belongsTo(Human, "human_id"),
      movie: this.belongsTo(Movie, "episode_id"),
      saved: this.morphOne(Saved, "id", "T", "movie_id").onDelete("cascade"),
    };
  }
 get saved_id() {
    return this.movie_id;
  }
}

class HumanSavedEpisode extends Model {
  static entity = "HumanSavedEpisode";
  static primaryKey = ["human_id", "episode_id"];

  static fields() {
    return {
      episode_id: this.uid(),
      human_id: this.uid(),
      added_at: this.attr(utcsec()),
      human: this.belongsTo(Spotify_Human, "human_id"),
      episode: this.belongsTo(Spotify_Episode, "episode_id"),
      saved: this.morphOne(Saved, "id", "T", "episode_id").onDelete("cascade"),
    };
  }
 get saved_id() {
    return this.episode_id;
  }
}

// and now the Saved:

class Saved<Item extends SavebleT> extends Model {
  static entity = "saved";

  static fields() {
    return {
      // this is like item id
      id: this.uid(),
      added_at: this.number(utcsec),
      T: this.string(""),
      item: this.morphTo(
        [HumanSavedMovie, HumanSavedEpisode],
        "id",
        "T",
        "saved_id"
      ),
    };

  declare id: string;
  /* type */
  declare T: SavebleT;
  declare item: Item extends "HumanSavedMovie"
    ? HumanSavedMovie
    : Item extends "HumanSavedEpisode"
    ? HumanSavedEpisode"
    : never;
}

The first issue I encountered is that all my HumanSavedX models had different primary keys. This does not work with polymorphic relationships because you can only specify one custom ownerKey. I could work around this by specifying the saved_id getter on HumanSavedMovie and HumanSavedEpisode. Maybe being able to pass a list of ownerKeys the same length as the first input to MorphTo is a good idea.

However, this trick does not work for relationships. This query does not work:

const saved = useRepo(Saved);
const items = saved.query().with("item", (item) => item.with("movie")).get()

This one gives the error that the movie relationship does not exist on HumanSavedEpisode.

and neither this query works:

const saved = useRepo(Saved);
const items = saved.query().with("item", (item) => item.with("episode")).get()

This one gives the error that the episode relationship does not exist on HumanSavedMovie.

---

One solution of course would be to rename movie and episode to item.
I could still use the composite primary key but it would be less clear. Also, this just moves the problem. My movie model happens to have an artists relationship. This relationship does not exist on episodes. This means I can not query the artists on the movie!

This error means that, yes you can link multiple entities to the same entity with polymorphic relationships. The relationships of the entities that you link to have to be quite similar for it to be useful.

Could we think about a way to not throw an error with polymorphic relationships and queries like this?

const items = saved.query().with("item", (item) => item.with("episode").with("movie")).get()

I would like to have a discussion about this.

[tintin10q]

One possible direction could be something like this:

const all_saved = computed(() =>
  saved
    .query()
    .with("saved_item", (saved_item) =>
      saved_item
        .whereHas("movie", (query) => query.with("artists"))
        .orWhereHas("episode", (query) => query.with("episode"))
    )
    .get()
);

Where you specify the relationships based on having certain other relationships.

[tintin10q]

Another solution could be to make fake relationships on other models. Like giving my episodes an artist relationship which will never have anything. But this does not seem very scalable.

[tintin10q]

Just pining so you see this. @CodeDredd

[CodeDredd]

Soon as i have time i look onto it.

[tintin10q]

Actually I think https://pinia-orm.codedredd.de/guide/model/single-table-inheritance is exactly what I need.