module ActiveRecord::UnionScope

gistfile1.rb

#https://gist.github.com/tlowrimore/5162327
#http://stackoverflow.com/a/15413611/270511

module ActiveRecord::UnionScope
  def self.included(base)
    base.send :extend, ClassMethods
  end
  
  module ClassMethods
    def union_scope(*scopes)
      id_column = "#{table_name}.id"
      
      if (sub_query = scopes.reject { |sc| sc.count == 0 }.map { |s| s.select(id_column).to_sql }.join(" UNION ")).present?
        where "#{id_column} IN (#{sub_query})"
      else
        none
      end
    end
  end
end

Just fixed this by counting the scopes that actually return something. The previous way failed sometimes.

Even better!

If my scopes accept arguments, will it work ??

this is correct but needs just small change:

This code:

 map { |s| s.select(id_column).to_sql }

should be replaced with this:

.map { |s|   "(#{s.select(id_column).to_sql})" }