Format the datetime string according to the precision of the datetime field.

Incompatible to rounding behavior between MySQL 5.6 and earlier.

In 5.5, when you insert `2014-08-17 12:30:00.999999` the fractional part
is ignored. In 5.6, it's rounded to `2014-08-17 12:30:01`:

http://bugs.mysql.com/bug.php?id=68760

activerecord/CHANGELOG.md

@@ -1,3 +1,14 @@
+*   Format the datetime string according to the precision of the datetime field.
+
+    Incompatible to rounding behavior between MySQL 5.6 and earlier.
+
+    In 5.5, when you insert `2014-08-17 12:30:00.999999` the fractional part
+    is ignored. In 5.6, it's rounded to `2014-08-17 12:30:01`:
+
+    http://bugs.mysql.com/bug.php?id=68760
+
+    *Ryuta Kamizono*
+
 *   Allow precision option for MySQL datetimes.
 
     *Ryuta Kamizono*

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -688,7 +688,7 @@ module ActiveRecord
 
         m.register_type(%r(datetime)i) do |sql_type|
           precision = extract_precision(sql_type)
-          Type::DateTime.new(precision: precision)
+          MysqlDateTime.new(precision: precision)
         end
 
         m.register_type(%r(enum)i) do |sql_type|
@@ -884,6 +884,22 @@ module ActiveRecord
         TableDefinition.new(native_database_types, name, temporary, options, as)
       end
 
+      class MysqlDateTime < Type::DateTime # :nodoc:
+        def type_cast_for_database(value)
+          if value.acts_like?(:time) && value.respond_to?(:usec)
+            result = super.to_s(:db)
+            case precision
+            when 1..6
+              "#{result}.#{sprintf("%0#{precision}d", value.usec / 10**(6 - precision))}"
+            else
+              result
+            end
+          else
+            super
+          end
+        end
+      end
+
       class MysqlString < Type::String # :nodoc:
         def type_cast_for_database(value)
           case value

activerecord/test/cases/adapters/mysql/datetime_test.rb

@@ -2,6 +2,9 @@ require 'cases/helper'
 
 if mysql_56?
   class DateTimeTest < ActiveRecord::TestCase
+    self.use_transactional_fixtures = false
+
+    class Foo < ActiveRecord::Base; end
 
     def test_default_datetime_precision
       ActiveRecord::Base.connection.create_table(:foos, force: true)
@@ -50,6 +53,20 @@ if mysql_56?
       assert_equal 4, mysql_datetime_precision('foos', 'updated_at')
     end
 
+    def test_formatting_datetime_according_to_precision
+      ActiveRecord::Base.connection.create_table(:foos, force: true) do |t|
+        t.datetime :created_at, precision: 0
+        t.datetime :updated_at, precision: 4
+      end
+      date = ::Time.utc(2014, 8, 17, 12, 30, 0, 999999)
+      Foo.create!(created_at: date, updated_at: date)
+      assert foo = Foo.find_by(created_at: date)
+      assert_equal date.to_s, foo.created_at.to_s
+      assert_equal date.to_s, foo.updated_at.to_s
+      assert_equal 000000, foo.created_at.usec
+      assert_equal 999900, foo.updated_at.usec
+    end
+
     private
 
     def mysql_datetime_precision(table_name, column_name)

activerecord/test/cases/adapters/mysql2/datetime_test.rb

@@ -2,6 +2,9 @@ require 'cases/helper'
 
 if mysql_56?
   class DateTimeTest < ActiveRecord::TestCase
+    self.use_transactional_fixtures = false
+
+    class Foo < ActiveRecord::Base; end
 
     def test_default_datetime_precision
       ActiveRecord::Base.connection.create_table(:foos, force: true)
@@ -50,6 +53,20 @@ if mysql_56?
       assert_equal 4, mysql_datetime_precision('foos', 'updated_at')
     end
 
+    def test_formatting_datetime_according_to_precision
+      ActiveRecord::Base.connection.create_table(:foos, force: true) do |t|
+        t.datetime :created_at, precision: 0
+        t.datetime :updated_at, precision: 4
+      end
+      date = ::Time.utc(2014, 8, 17, 12, 30, 0, 999999)
+      Foo.create!(created_at: date, updated_at: date)
+      assert foo = Foo.find_by(created_at: date)
+      assert_equal date.to_s, foo.created_at.to_s
+      assert_equal date.to_s, foo.updated_at.to_s
+      assert_equal 000000, foo.created_at.usec
+      assert_equal 999900, foo.updated_at.usec
+    end
+
     private
 
     def mysql_datetime_precision(table_name, column_name)