Datetime field saves nil value when saving a string

I have a problem with Datetime field in my Rails app. I have a validation that should accept valid Datetime, and allows null or blank values (code is from my question yesterday):

  include ActiveModel::Validations
  class DateValidator < ActiveModel::EachValidator
    def validate_each(record,attribute,value)
      record.errors[attribute] << "must be a valid datetime" unless ((DateTime.parse(value) rescue nil))
    end
  end
  validates :datetime_field :date => true, :allow_nil => true, :allow_blank => true

However, when set datetime_field to some string, my model overrides the previous value of datetime_field and sets it to nil (in rails console I get the following:

object.update_attributes("datetime_field" => "Now")
true
object.datetim开发者_Go百科e_field.nil?
true

How to stop setting my datetime field to nil after updating with string, and at the same time keep being able to blank this field explicitly?

---

You validations are strange: datetime_field should be a date and at the same time it can be nil or blank. But nil or blank can't be date. So your validation should sounds like: datetime_field should be DATE or BLANK or NIL:

include ActiveModel::Validations
class DateOrBlankValidator < ActiveModel::EachValidator
  def validate_each(record,attribute,value)
    record.errors[attribute] << "must be a valid datetime or blank" unless value.blank? or ((DateTime.parse(value) rescue nil))
  end
end
validates :datetime_field :date_or_blank => true

UPD

Just for notice. nil.blank? => true. So you never need to validate something if it is nil if you are checking if it is blank?. Blank? will return true for empty objects and for nil objects: "".blank? => true, nil.blank? => true

---

Is your datetime_field marked as datetime in your migration?

If so datetime_field is being set to nil becouse string you've passed isn't valid datetime string. Try doing this instead:

object.update_attributes("datetime_field" => "2010-01-01")
true

Is suppose then

object.datetime_field

should return

2010-01-01