Ability to use union on activerecord relations. Put: include ActiveRecord::UnionScope in the model.

activerecord_union.rb

module ActiveRecord
  module UnionScope
    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
ActiveRecord::Base.send(:extend, ActiveRecord::UnionScope)

Less problems and easier to follow:

    def union_scope(*scopes)
      scopes.inject(self) { |all, scope| all.or(where(id: scope)) }
    end