Add select_except

https://github.com/SeaQL/sea-orm/discussions/3026#discussioncomment-16994303

I’d probably avoid #[lazy] on the normal Model.

If file is part of the model type but not actually loaded, then every access needs some hidden state like “loaded / not loaded / loading failed”. That makes the model less predictable and easy to misuse.

For this case I’d keep it explicit:

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

let rows = Entity::find()
    .select_only()
    .column(Column::Id)
    .column(Column::Name)
    .into_model::<EntryWithoutFile>()
    .all(db)
    .await?;

Then load the blob only when needed:

let file: Option<Vec<u8>> = Entity::find_by_id(id)
    .select_only()
    .column(Column::File)
    .into_tuple()
    .one(db)
    .await?;

So imo a defer() / select_except() style API would fit SeaORM better than lazy fields on Model.

Something like this would be really nice:

Entity::find()
    .select_except([Column::File])
    .all(db)
    .await?;

For bigger files, I’d also consider moving the bytes to object storage / filesystem and keeping only metadata + path/key in DB. But if the files are small and DB storage is intentional, explicit projection + a helper method is probably the cleanest workaround for now.

Maybe you can wrap it like:

impl EntryWithoutFile {
    async fn load_file(&self, db: &DatabaseConnection) -> Result<Option<Vec<u8>>, DbErr> {
        Entity::find_by_id(self.id)
            .select_only()
            .column(Column::File)
            .into_tuple()
            .one(db)
            .await
    }
}

Not as magic as #[lazy], but much clearer and avoids accidentally fetching blobs in list queries.

Start at the existing Entity::find().select_only() query-selection API and trace how selected columns are represented and applied. Add a select_except([Column::File])-style entry point whose completed behavior excludes the requested column while preserving the remaining query results; verify it with the project’s existing query-selection tests.