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

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})" }