Function to create a subselect so it can be used in Diesel's select()

create_subselect.rs

use diesel::prelude::*;
use diesel::backend::Backend;
use diesel::expression::{Expression, AsExpression, Selectable, ValidGrouping};
use diesel::query_builder::{AstPass, Query, QueryFragment};
use diesel::{QueryResult};
use diesel::pg::Pg;
use diesel::sql_types::{SingleValue};
use crate::schema::{order_order, order_status};

pub struct Subquery<T> {
    subquery: T,
    alias: String,
}

impl<T> QueryFragment<Pg> for Subquery<T>
    where
        T: QueryFragment<Pg>,
{
    fn walk_ast<'b>(&'b self, mut out: AstPass<'_, 'b, Pg>) -> QueryResult<()> {
        self.subquery.walk_ast(out.reborrow())?;
        out.push_sql(" AS ");
        out.push_identifier(self.alias.as_str()).expect("Error adding alias");
        Ok(())
    }
}

impl<T> Expression for Subquery<T>
where T: Expression
{
    type SqlType = T::SqlType;
}

impl<T> AppearsOnTable<order_order::table> for Subquery<T>
where T: Expression
{}

impl<T> SelectableExpression<order_order::table> for Subquery<T>
where T: Expression
{}

impl<T, GB> ValidGrouping<GB> for Subquery<T>
where T: Expression + ValidGrouping<GB>
{
    type IsAggregate = T::IsAggregate;
}

impl<T, DB: Backend> Selectable<DB> for Subquery<T>
where T: QueryFragment<DB> + Expression + Selectable<DB>
{
    type SelectExpression = order_status::id;

    fn construct_selection() -> Self::SelectExpression {
        order_status::id
    }
}


pub fn get_subquery_for_select<T>(subquery: T, alias: String) -> Subquery<T> {
    Subquery {
        subquery,
        alias
    }
}