SQL查询到Ruby on Rails

Question

I am wandering if you can help me with this.
I have this query on SQL and I need to do it in my rails app.
These query's represent the same result and works good.

SELECT DISTINCT ON (i.id)   
i.id, i.den_cont, c.date_expired  
FROM Items i  
JOIN Calibrations c ON c.item_id = i.id  
ORDER BY i.id, date_expired desc  

and

select      codigo, numero, den_cont, date_expired  
from        items  
join calibrations c  
on      items.id = c.item_id  
inner join (select item_id, MAX(date_expired) as FECHA  
from calibrations  
group by item_id) as i2  
on c.item_id = i2.item_id and c.date_expired = i2.FECHA  

I have this, I need to change it to do the above..

@items = Item.joins(:calibrations).where('calibrations.date_expired <= ?', '2014/09/01')

Answer 1

Does something like this help?

Items.join(:calibrations)
     .where('calibrations.date_expired <= ?', '2014-09-01')
     .order('items.id')
     .group('items.id', 'items.den_cont')
     .select('items.id', 'items.den_cont', 'MAX(calibrations.date_expired)')

I assume items.id is the primary key and then the second group argument is, just so you can select the den_cont column.

EDIT: This is also an option,

Items.join(:calibrations)
     .where('calibrations.date_expired <= ?', '2014-09-01')
     .order('items.id')
     .order('calibrations.date_expired DESC')
     .select('DISTINCT ON (items.id) items.id, items.den_cont, calibrations.date_expired AS colaboration_expired')