use itertools::Itertools;

#[derive(Debug, PartialEq)]
struct User {
    id: i32,
}

#[derive(Debug)]
struct Post {
    id: i32,
    uid: i32,
    title: &'static str,
}

fn main() {
    let query = vec![
        (User { id: 1 }, Post { id: 1, uid: 1, title: "a" }),
        (User { id: 1 }, Post { id: 2, uid: 1, title: "b" }),
        (User { id: 2 }, Post { id: 2, uid: 2, title: "a" }),
    ];
    let users = query.iter()
        .group_by(|(user, _post)| user)
        .into_iter()
        .map(|(user, group)| {
            (user, group.map(|(_user, post)| post).collect::<Vec<_>>())
        })
        .collect::<Vec<_>>();
    dbg!(users);
}