as_guery() should not be needed to pass a table to exists

[alatiera]

Compiler Errors unless the method .as_query() on foo::table is used.

pub fn is_episodes_populated() -> Result<bool, Error> {
    use schema::episode::dsl::*;

    let db = connection();
    // Using r2d2
    let con = db.get()?;

    select(exists(episode))
        .get_result(&con)
        .map_err(From::from)
}

Error: https://paste.gnome.org/prsogzgak

Changing the select to select(exists(episode.as_query())) works, but @sgrif said
that ideally the explicit .as_query() should not be required.

select(exists(episode.as_query()))
    .get_result(&con)
    .map_err(From::from)