How to do “where exists” in Arel

问题

How do you do a query that includes a "where exists" in Arel? For example on a query like this to show all the suppliers with at least one order:

SELECT *
FROM suppliers
WHERE EXISTS
  (SELECT *
    FROM orders
    WHERE suppliers.supplier_id = orders.supplier_id);

I see "exists" in the Arel docs http://rubydoc.info/gems/arel/2.0.7/Arel/Nodes/Exists but I'm having trouble using it.

回答1:

Here you go:

suppliers= Supplier.arel_table
orders= Order.arel_table
suppliers_with_orders = Supplier.where(
                          Order.where(orders[:supplier_id]
                                        .eq(suppliers[:id])).exists).to_sql =>
"SELECT `suppliers`.* FROM `suppliers` 
 WHERE (EXISTS (SELECT `orders`.* 
                FROM `orders` 
                WHERE `suppliers`.`id` = `orders`.`supplier_id`))"

Though, an inner join would do this in a more simple - and eventually less performant - way :

Supplier.joins :orders

回答2:

I think that you can neatly use a counter_cache for this one :

http://asciicasts.com/episodes/23-counter-cache-column

来源：https://stackoverflow.com/questions/6207139/how-to-do-where-exists-in-arel