datetime associations work only with hour-containing input

I don't know how Rails create accessors based on DBs column type, but accessors for SQL datetime time act weird.

The problem: assigning string (for example from a form) to datetime value (like begins_at) works good with full ISO-format string ("YYYY-MM-DD hh:mm"), but given a string without an hour ("YYYY-MM-DD") sets the field to nil.

>> f.happened_at = "2004-05-06 11:45"
=> "2004-05-06 11:45"
>> f.happened_at
=> Thu May 06 11:45:00 +0200 2004
>> f.happened_at = "2004-05-06"
=> "2004-05-06"
>> f.happened_at
=> nil

(Which of course automatically breaks validates_date_time plugin, but this is irrelevant ATM).

The workaround: custom accessors work fine when using to_datetime on string, also on one without hours:

class CalendarEvent < ActiveRecord::Base
  def begins_at=(datetime_str)
    write_attribute(:begins_at, datetime_str.to_datetime)
  end
end

Proposed solution: make datetime field accessors preprocess the value given with .to_datetime. Works like charm for Time, Date, Datetime and "ill-formatted" (i.e. ISO without an hour) strings described above.

I'd gladly do it myself (or at least cover it with a plugin), but don't know where to dig, i.e. which method of which class & module to overload.