class ActiveRecord::Associations::HasManyAssociation
  def sum(*args, &block)
    return calculate(:sum, *args) if block.nil?
    load_target
    @target.sum(*args, &block)
  end
end

class ActiveRecord::Associations::HasManyThroughAssociation
  def sum(*args, &block)
    return calculate(:sum, *args) if block.nil?
    load_target
    @target.sum(*args, &block)
  end
end

class ActiveRecord::Associations::HasAndBelongsToManyAssociation
  def sum(*args, &block)
    return calculate(:sum, *args) if block.nil?
    load_target
    @target.sum(*args, &block)
  end
end