Function aggregate 

pub fn aggregate<'outer, S, F, R>(f: F) -> R
where
    F: for<'inner> FnOnce(&mut Aggregate<'outer, 'inner, S>) -> R,

Perform an aggregate that returns a single result for each of the current rows.

One can filter the rows in the aggregate based on values from the outer query. See the documentation for Aggregate for more information.

#[schema(Site)]
pub mod vN {
    pub struct Review {
        #[index]
        pub book: rust_query::TableRow<Book>,
        pub rating: f64,
    }
    pub struct Book {
        pub name: String
    }
}
use v0::*;

Database::new(Config::open_in_memory()).transaction(|txn| {
    let books = txn.query(|rows| {
        let book = rows.join(Book);
        let rating = aggregate(|aggr| {
            let review = aggr.join(Review.book(&book));
            // books without reviews will get a rating of 0.0
            aggr.avg(&review.rating).unwrap_or(0.0)
        });
        // top 10 highest rated books
        rows.order_by()
            .desc(rating)
            .into_iter(book)
            .take(10)
    });
});