Fix occasional microsecond conversion inaccuracy

ActiveRecord::ConnectionAdapters::Column#microseconds did an unnecessary
conversion to from Rational to float when calculating the integer number
of microseconds. Some terminating decimal numbers in base10 are
repeating decimal numbers in base2 (the format of float), and
occasionally this causes a rounding error.
Patch & explanation originally from Logan Bowers.

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Fix Column.microseconds and Column.fast_string_to_date to avoid converting
+    timestamp seconds to a float, since it occasionally results in inaccuracies
+    with microsecond-precision times. Fixes #7352.
+
+    *Ari Pollak*
+
 *   Raise `ArgumentError` if list of attributes to change is empty in `update_all`.
 
     *Roman Shatsov*

activerecord/lib/active_record/connection_adapters/column.rb

@@ -208,7 +208,7 @@ def value_to_decimal(value)
           # '0.123456' -> 123456
           # '1.123456' -> 123456
           def microseconds(time)
-            ((time[:sec_fraction].to_f % 1) * 1_000_000).to_i
+            time[:sec_fraction] ? (time[:sec_fraction] * 1_000_000).to_i : 0
           end
 
           def new_date(year, mon, mday)
@@ -233,7 +233,7 @@ def fast_string_to_date(string)
           # Doesn't handle time zones.
           def fast_string_to_time(string)
             if string =~ Format::ISO_DATETIME
-              microsec = ($7.to_f * 1_000_000).to_i
+              microsec = ($7.to_r * 1_000_000).to_i
               new_time $1.to_i, $2.to_i, $3.to_i, $4.to_i, $5.to_i, $6.to_i, microsec
             end
           end

activerecord/test/cases/base_test.rb

@@ -231,6 +231,7 @@ def test_preserving_time_objects
       assert_equal 11, Topic.find(1).written_on.sec
       assert_equal 223300, Topic.find(1).written_on.usec
       assert_equal 9900, Topic.find(2).written_on.usec
+      assert_equal 129346, Topic.find(3).written_on.usec
     end
   end
 

activerecord/test/fixtures/topics.yml

@@ -25,7 +25,7 @@ third:
   id: 3
   title: The Third Topic of the day
   author_name: Carl
-  written_on: 2005-07-15t15:28:00.0099+01:00
+  written_on: 2012-08-12t20:24:22.129346+00:00
   content: I'm a troll
   approved: true
   replies_count: 1