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I have a model object in my Rails application, let's call it Picture.

The pictures table in database has a primary key column called id because Rails create them automatically according to convention, this column is just 1, 2, 3, 4.....

But I need another column called custom_id to record its custom ID, I want its value to be

  • PIC2015-0001, PIC2015-0002, PIC2015-0003 for year 2015
  • PIC2018-0001, PIC2018-0002, PIC2018-0003 for year 2018
  • and so on.

Basically PICYYYY-NUM_FOR_YEAR, where YYYY is the year it is created, and NUM_FOR_YEAR is the number this record is created (first record created in a year will have 0001, second will have 0002, and so on... This number will restart for new record each new year)

I want to implement this by using the after_create hook and look up its created_at time stamp to get the year YYYY, and infer the NUM_FOR_YEAR by looking at last Picture with non-empty custom_id. But I'm worried about synchronisation issue. What if the below happen (or will it happen at all?):

The below events happen in the time order listed

  • Picture 1 created, after_create called and finished
  • Picture 2 created
  • Picture 3 created
  • Picture 2 after_create called - fetch Picture 1's custom_id
  • Picture 3 after_create called - fetch Picture 1's custom_id
  • Picture 2 after_created finished
  • Picture 3 after_created finished - this will have duplicate ID, which is wrong

What is the simplest way to solve this problem?

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  • Why do you need to "fetch Picture 1's custom_id"?... Maybe you can use the callback after_create_commit, in there get the ID generated from your DB, and from there update the custom_id Read here Commented Jul 31, 2018 at 1:53
  • Wouldn't using after_create_commit cause the same synchronisation problem? (Or it won't?) Also you said use the ID generated from your DB, but I want to restart from 0001 every year. Can you clarify your implementation so I can understand why it might address the problem I have? Commented Jul 31, 2018 at 2:27
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    I'm sorry, you're right, I forgot about that (restarting the count every year). Maybe, use a unique index at the DB level, at least this way you guarantee not to repeat a custom_id, the DB will raise an error if you try to duplicate the value. As far as preventing the sync problem, at the application level, maybe with a unique validation in your model (then again this actually don't prevent the sync problem, but at least you are safe with the DB constraint) Commented Jul 31, 2018 at 2:39
  • Thank you, the DB level unique constraint is really helpful Commented Jul 31, 2018 at 3:12
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    In this case and assuming also no concurrent create, just pick the year from created_at and count records where created_at this_year, build the string and save to custom_id. How fast is the data saving? You reserved 4 digits for a whole year, so it is 10000 records = 1,15 per hour. I wouldn't worry of concurrency, but better set unique constraint as validation. Commented Jul 31, 2018 at 6:42
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