How do I write a UNION chain with ActiveRelation?

Question

I need to be able to chain an arbitrary number of sub-selects with UNION using ActiveRelation.

I\'m a little confused by the ARel implementation of this, si

Answer 1

Because of the way the ARel visitor was generating the unions, I kept getting SQL errors while using Arel::Nodes::Union. Looks like old-fashioned string interpolation was the only way to get this working.

I have a Shift model, and I want to get a collection of shifts for a given date range, limited to five shifts per day. This is a class method on the Shift model:

def limit_per_day(options = {})
  options[:start]   ||= Date.today
  options[:stop]    ||= Date.today.next_month
  options[:per_day] ||= 5

  queries = (options[:start]..options[:stop]).map do |day|

    select{id}.
    where{|s| s.scheduled_start >= day}.
    where{|s| s.scheduled_start < day.tomorrow}.
    limit(options[:per_day])

  end.map{|q| "( #{ q.to_sql } )" }

  where %{"shifts"."id" in ( #{queries.join(' UNION ')} )}
end

(I am using Squeel in addition to ActiveRecord)

Having to resort to string-interpolation is annoying, but at least the user-provided parameters are being sanitized correctly. I would of course appreciate suggestions to make this cleaner.