终于找到了解决办法。谢谢Slack Overflow是一只很棒的橡胶鸭。

# models/product.rb

class Product < ApplicationRecord
  scope :product_number_one_of, lambda { |product_numbers|
    product_numbers = product_numbers.split(',').map(&:strip) if product_numbers.is_a?(String)
    where(product_number: product_numbers)
  }

  def self.ransackable_scopes(*)
    [:product_number_one_of]
  end
end

# admin/products.rb

ActiveAdmin.register Product do
  # ...
  filter :product_number_one_of, as: :string, label: 'Product Number'
  # ...
end