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

1 files changed, 17 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index ac62fb9825..d8431372ee 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/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