Function nth_value 

pub fn nth_value<T: SqlType + SingleValue + IntoNullable<Nullable: SingleValue>, value, n>(
    value: value,
    n: n,
) -> nth_value<T, <value as AsExpression<T>>::Expression, <n as AsExpression<Integer>>::Expression>
where
    value: AsExpression<T>,
    n: AsExpression<Integer>,

Value of argument from N-th row of window frame

Returns value evaluated at the row that is the n’th row of the window frame (counting from 1); returns NULL if there is no such row.

This function must be used as window function. You need to call at least one of the methods WindowExpressionMethods from to use this function in your SELECT clause. It cannot be used outside of SELECT clauses.

let res = posts
    .select((title, user_id, nth_value(id, 2).partition_by(user_id)))
    .load::<(String, i32, Option<i32>)>(connection)?;
let expected = vec![
    ("My first post".to_owned(), 1, Some(2)),
    ("About Rust".into(), 1, Some(2)),
    ("My first post too".into(), 2, None),
];
assert_eq!(expected, res);