The problem only manifests itself if the sub-second part is stored with 4 or more significant digits. So if you only store and retrieve milliseconds (and thats the way I read your code) you should be home safe.
On 15/01/2010, at 15.56, Ken Collins wrote: > > I plan on taking a look at this too. I think I had to solve it in the SQL > Server adapter in my own way since it only stores milliseconds. If you care > to look at our code, here are a few key sections in the tests. > > http://github.com/rails-sqlserver/2000-2005-adapter/blob/master/test/cases/adapter_test_sqlserver.rb#L271 > http://github.com/rails-sqlserver/2000-2005-adapter/blob/master/lib/active_record/connection_adapters/sqlserver_adapter.rb#L328 > > > > On Jan 15, 2010, at 9:45 AM, Jacob Lauemøller wrote: > >> Thanks for the response -- we use PostgreSQL which does store all six >> microsecond digits. >> >> Jacob >> >> >> On 15/01/2010, at 15.41, Chris Cruft wrote: >> >>> I gave up on that kind of resolution when I found that MySQL doesn't >>> support it! I'll try to test the patch though. >>> >>> -Chris >>> >>> On Jan 14, 10:20 am, Jacob Lauemøller <[email protected]> >>> wrote: >>>> Hi all, >>>> >>>> The microsecond handling in >>>> ActiveRecord::ConnectionAdapters::Column#fast_string_to_time and >>>> ActiveRecord::ConnectionAdapters::Column#microseconds fail for some values. >>>> >>>> In slightly more than 1% of all possible 6-digit cases, writing a >>>> timestamp to a database column and then reading it back in results in a >>>> different value being returned to the program. >>>> >>>> So, for instance, saving the timestamp >>>> >>>> 2010-01-12 12:34:56.125014 >>>> >>>> and then loading it again from the database yields >>>> >>>> 2010-01-12 12:34:56.125013 >>>> >>>> The problem occurs when the value read is converted from string form to a >>>> Ruby timestamp, so it is largely database independent (the exception being >>>> drivers that override the methods, or databases that don't support >>>> timestamps at all). >>>> >>>> The underlying problem is the use of to_i to convert from floats to ints >>>> inside the affected methods. As you know, to_i simply truncates the result >>>> and in some cases this causes rounding errors introduced by inherent >>>> inaccuracies in the multiplication operations and decimal representation >>>> to bubble up and affect the least significant digit. >>>> >>>> Here's a simple test that illustrates the problem: >>>> >>>> converted = >>>> ActiveRecord::ConnectionAdapters::Column.send('fast_string_to_time', >>>> "2010-01-12 12:34:56.125014") >>>> assert_equal 125014, converted.usec >>>> >>>> This test case (and a similar one for #microseconds) fail on plain vanilla >>>> Rails 2.3.5. >>>> >>>> I guess the best solution would be to change the ISO_DATETIME regex used >>>> to extract the microsecond-part from timestamps to not include the decimal >>>> point at all and then avoid the to_f and subsequent floating point >>>> multiplication completely inside the failing methods. However, these >>>> regexes are defined as constants on >>>> ActiveRecord::ConnectionAdapters::Column::Format and therefore publicly >>>> available, so the impact of changing these is difficult to ascertain. >>>> >>>> A simpler solution is to use round() instead of to_i to convert from the >>>> intermediate floating point result to int. This works (I have verified >>>> that the precision is sufficient for all possible 6-digit cases) but is >>>> about 15% slower than the current method. A small price to pay for >>>> correctness, in my opinion. >>>> >>>> I have attached a tiny patch (against 2.3.5) that switches the code to >>>> using round() and a test case that verifies that the method works for a >>>> few problematic cases that fail without the patch. >>>> >>>> I have also created a Lighthouse ticket #3693: >>>> >>>> https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/3... >>>> >>>> Could some of you please take a look and see if the patch is acceptable >>>> and maybe carry it into the code base? >>>> >>>> Cheers >>>> Jacob >>>> >>>> fix_microsecond_conversion.diff >>>> 4KViewDownload >>>> >>>> >>> -- >>> You received this message because you are subscribed to the Google Groups >>> "Ruby on Rails: Core" group. >>> To post to this group, send email to [email protected]. >>> To unsubscribe from this group, send email to >>> [email protected]. >>> For more options, visit this group at >>> http://groups.google.com/group/rubyonrails-core?hl=en. >>> >>> >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Ruby on Rails: Core" group. >> To post to this group, send email to [email protected]. >> To unsubscribe from this group, send email to >> [email protected]. >> For more options, visit this group at >> http://groups.google.com/group/rubyonrails-core?hl=en. >> >> > > -- > You received this message because you are subscribed to the Google Groups > "Ruby on Rails: Core" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/rubyonrails-core?hl=en. > >
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