Rails: How to get objects with at least one child?

Question

After googling, browsing SO and reading, there doesn\'t seem to be a Rails-style way to efficiently get only those Parent objects which have at leas

Answer 1

You just want an inner join with a distinct qualifier

SELECT DISTINCT(*) 
FROM parents
JOIN children
ON children.parent_id = parents.id

This can be done in standard active record as

Parent.joins(:children).uniq

However if you want the more complex result of find all parents with no children you need an outer join

Parent.joins("LEFT OUTER JOIN children on children.parent_id = parent.id").
where(:children => { :id => nil })

which is a solution which sux for many reasons. I recommend Ernie Millers squeel library which will allow you to do

Parent.joins{children.outer}.where{children.id == nil}