Question

I have a table of Groups that have a “capacity” column and has a has_many relationship with Enrollments. I want to be able to find Groups whose count of enrollments is less than it’s capacity, so using ActiveRecord + Ruby I can do this:

Group.all.select {|g| g.enrollments.count < g.capacity }.first

But it seems like there should be a way to do this in SQL, I just don’t know how. Any ideas?

Answer 1

The pure SQL way of doing this would be

select groups.* from groups
    inner join enrollments on enrollments.group_id = groups.id
group by groups.id
having count(*) < capacity

Or in activerecord

Group.joins(:enrollments).group('groups.id').having('count(*) < capacity)

A counter cache with an index on the counter column will be faster though, although obviously you have to not create enrollments behind acriverecord’s back.