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 <jacob.lauemoel...@iteray.com>
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
>
>
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