An Unsanctioned Union

activerecord3_union.rb

module ActiveRecordScopeExtensions
  def self.union(left, right)
    active_record_klass = left.klass
    
    arel_union = [left, right].map do |scope|
      scope.select("#{active_record_klass.table_name}.id")
    end.reduce(&:union)
    
    temporary_table_name = TemporaryTableName.new([active_record_klass.table_name, "union"].join("_")).to_s
    active_record_klass.joins("INNER JOIN (#{arel_union.to_sql}) as #{temporary_table_name} ON #{temporary_table_name}.id = #{active_record_klass.table_name}.id")
  end

  class TemporaryTableName
    def initialize(prefix, random = SecureRandom.hex)
      @prefix = prefix
      @random = random
    end

    def to_s
      [prefix, random].join("_")
    end

    private

    attr_reader :random, :prefix
  end
end