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Editorial Team
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Asked: 2026-05-29T07:05:23+00:00

I’m thinking through how to develop a validation on my Rails app that essentially

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I’m thinking through how to develop a validation on my Rails app that essentially checks to make sure that the credit card used for any given transaction by any user is unique in our system, such that all credit card may be used to purchase an item only once across the entire application for all users, for all time.

The thinking behind this restriction is that this app will sometimes run time-sensitive promotional deals, and we want to do our best to institute a “one purchase per credit card” system for these deals.

I was thinking of hashing the credit card number and just storing that hash in the db, then cross-referencing it at the time of each new purchase (so my payment gateway keeps the actual number, and I just keep a hash in the DB), but on further research, this seems like a bad idea.

So I’m back to the drawing board and looking for new ideas. Anyone know a good approach to this problem, while keeping as PCI-compliant as I can be?

I’m developing with Rails 3 and using ActiveMerchant to integrate with my payment gateway, Authorize.net, if that helps at all.

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1 Answer

  1. Editorial Team

    I think you are looking in the wrong direction. I would just check last 4 of card, ip and shipping addresses. The risks of storing that data versus the damage if a small number of users gamed the last 4 & ip solution is not worth it. (He says not knowing the nature of the purchases.)

    Since address isn’t collected…First 4, Last 4 and 4 Digit Expiration (all hashed of course) should provide the uniqueness you need to ensure that card was only used once.

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