Question

I have the following models in a Rails 3 app, and select requirement:

class Item < AR
has_many :holdings


class Holding < AR
belongs_to :item

The Holding model has an ‘active’ boolean value.

I wish to find each Item that has 0 ‘active’ holdings ( it may have any number of associated holdings ), I’ve tried quite a few combinations.

SELECT * from items JOIN
(SELECT holdings.item_id, count(ifnull(item_id,0)) AS hcount FROM holdings
WHERE holdings.active = "t"
GROUP BY holdings.item_id
HAVING hcount = 0)
ON items.id = holdings.item_id

but this will only return counts that are greater than 0.

Can anyone point me in the right direction?

Answer 1

Don’t use count when you mean any!

Use an not exists clause.

SELECT * from items i
where not exists(select holdings.item_id 
             from holdings 
             where holdings.active = 't' 
               and holdings.item_id = i.item_id)

This statement in english says give me all the rows from items where there are no matching rows in holdings.