Assigning value to serialized attribute causes SerializationTypeMismatch

Question

I find it surprising that this has not been asked yet, so I am hoping I am doing something fundamentally wrong and help will arrive soon. I have this

create_table "foo", :force => true do |t|
  t.text     "bar",                                     :null => false
  ...
end

class Foo < AR::Base
   serialize :bar, Bar
end

class Bar
   def initialize(hsh)
     @data = hsh # and some more code...
   end
end

Now once I add

class Foo < AR::Base
  before_validation :get_bar, :on => :create
  def get_bar
    self.bar = Bar.new({})
  end
end

I am not able to create any Foo objects because the assignment to Bar raises SerializationTypeMismatch. I have debugged this and found out that the value of bar is by default an empty string and AR validates that and fails.

The question is how to get rid of it.