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Editorial Team
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Asked: 2026-05-27T15:05:56+00:00

In my application I have: config.time_zone = ‘Warsaw’ A strange issue I have, is

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In my application I have: config.time_zone = 'Warsaw'

A strange issue I have, is that it seems like Rails are having problems with comparision of datetime fields.
If I change the datetime 1 hour back (and Warsaw is currently in timezone +0100), Rails won’t update the database, even if the field has changed. However, if I change the field once again, then the update will go to the database.

Example:

(Rails 3.1.0, ruby-1.9.2-p290, fresh rails app):

$ rails g model User starts_at:datetime
$ rake db:migrate
$ rails c
Loading development environment (Rails 3.1.0)
ruby-1.9.2-p290 :001 > u = User.create({:starts_at => "2011-01-01 10:00"})
SQL (21.3ms)  INSERT INTO "users" ("created_at", "starts_at", "updated_at") VALUES (?, ?, ?)  [["created_at", Tue, 13 Dec 2011 11:32:50 CET +01:00], ["starts_at", Sat, 01 Jan 2011 10:00:00 CET +01:00], ["updated_at", Tue, 13 Dec 2011 11:32:50 CET +01:00]]
 => #<User id: 1, starts_at: "2011-01-01 09:00:00", created_at: "2011-12-13 10:32:50", updated_at: "2011-12-13 10:32:50"> 
ruby-1.9.2-p290 :002 > u.starts_at
 => Sat, 01 Jan 2011 10:00:00 CET +01:00 # datetime created
ruby-1.9.2-p290 :003 > u.starts_at = "2011-01-01 09:00:00" # new datetime with one hour back
 => "2011-01-01 09:00:00" 
ruby-1.9.2-p290 :004 > u.starts_at
 => Sat, 01 Jan 2011 09:00:00 CET +01:00 # changed datetime
ruby-1.9.2-p290 :005 > u.save 
 => true 
ruby-1.9.2-p290 :006 > u.starts_at = "2011-01-01 09:00:00"
 => "2011-01-01 09:00:00" 
ruby-1.9.2-p290 :007 > u.save
   (0.3ms)  UPDATE "users" SET "starts_at" = '2011-01-01 08:00:00.000000', "updated_at" = '2011-12-13 10:33:17.919092' WHERE "users"."id" = 1
 => true

I’ve tested it in this fresh app, because I have a problem with this in larger application. What is going on? I’ve tried to browse the Rails code, tried to re-copy the relevant code ‘by-hand’ in console (like update, assign_attributes, even checked time_zone_conversion) and it worked, but not in ‘real world’..

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1 Answer

  1. Editorial Team

    looks like you stumbled on a similar issue.

    The problem appears to be here:
    https://github.com/rails/rails/blob/3-1-stable/activerecord/lib/active_record/attribute_methods/dirty.rb#L62

    When rails it testing if the value was changed it compares old & new:

    old = From cache (which is Time in your current timezone)

    new = Time in UTC (+00:00) as saved in the database

    If the difference in time is the UTC offset, the above erroneously succeeds (luckly the new cached value holds the intended change).

    The next save/update compares with the new (and correct) cached value and marks the field as changed.

    EDIT:

    Done some tests, this works well for me:

    https://github.com/rails/rails/blob/3-1-stable/activerecord/lib/active_record/attribute_methods/time_zone_conversion.rb#L50

    Change

    write_attribute(:#{attr_name}, original_time)

    to

    write_attribute(:#{attr_name}, time.in_time_zone('UTC').to_s)

    Boris

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