CODEWITHSUNDEEP
ruby-on-rails-3activerecord
mohamed-ibrahim
mohamed-ibrahim

Reputation: 11137

Activrecord timestamp wrong assignment

for the following:

         updated_at         |         created_at         |   id    
----------------------------+----------------------------+---------
 2016-08-26 12:33:35.900201 | 2016-08-25 12:33:13.782502 | 2951380
 2016-08-26 12:33:35.916025 | 2016-08-25 12:33:13.781838 | 2951379
 2016-08-25 12:33:13.684854 | 2016-08-25 12:33:13.684854 | 2951377
 2016-08-25 12:33:13.684753 | 2016-08-25 12:33:13.684753 | 2951378
 2016-08-25 12:33:13.652293 | 2016-08-25 12:33:13.652293 | 2951376
 2016-08-26 12:32:59.669535 | 2016-08-25 12:33:13.589147 | 2951375
 2016-08-26 12:32:59.680676 | 2016-08-25 12:33:13.556841 | 2951374
 2016-08-26 12:32:59.559429 | 2016-08-25 12:33:13.496964 | 2951373
 2016-08-26 12:32:59.573863 | 2016-08-25 12:33:13.461594 | 2951372
 2016-08-26 12:31:10.338129 | 2016-08-25 12:33:13.400724 | 2951371

ID 2951378 has an earlier created_at (and updated_at) than the 2951377 record!

Anyone have any idea how that may happen, this records inserted by Queue worker handler.

Upvotes: 0

Views: 27

Answers (2)

mohamed-ibrahim
mohamed-ibrahim

Reputation: 11137

it looks like Rails calculates the timestamps, rather than relying on the database to do so

https://github.com/rails/rails/blob/55dfa009769962367c58563480c9f776ae0f53ea/activerecord/lib/active_record/timestamp.rb#L120

so it makes sense that this kind of situation can happen when multiple workers are saving records at the same time.

Upvotes: 0

Mark Bolusmjak
Mark Bolusmjak

Reputation: 24409

Imagine several transactions that are occurring simultaneously. They all need auto-generated ids. But the database cannot reserve the same ids for each transaction, because if they all succeed, they will override each other on commit.

So, each transaction gets its own set of auto-incremented values. Transaction A might start before transaction B, and get some ids allocated, but then B finishes first and it's larger IDs get saved with an earlier time.

It's not a sign of any error. It is a reminder that you should never assume the order of auto-generated IDs correlates to the sequence of events in a DB.

Upvotes: 1

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