build_association nullifies existing model object in has_one association

I have:

class Student < ActiveRecord::Base

has_one :language_grade

end

class LanguageGrade < ActiveRecord::Base

belongs_to :student

end

I do:

>> s=Student.create(:name=>"Norm")

=> #<Student id: 1, name: "Norm", created_at: "2009-04-06 13:43:09", updated_at: "2009-04-06 13:43:09">

>> s.language_grade

=> nil

>> s.create_language_grade(:score=>88)

=> #<LanguageGrade id: 1, student_id: 1, score: 88, created_at: "2009-04-06 13:43:46", updated_at: "2009-04-06 13:43:46">

>> LanguageGrade.all

=> [#<LanguageGrade id: 1, student_id: 1, score: 88, created_at: "2009-04-06 13:43:46", updated_at: "2009-04-06 13:44:13">]

>> s.build_language_grade(:score=>44)

=> #<LanguageGrade id: nil, student_id: 1, score: 44, created_at: nil, updated_at: nil>

>> LanguageGrade.all

=> [#<LanguageGrade id: 1, student_id: nil, score: 88, created_at: "2009-04-06 13:43:46", updated_at: "2009-04-06 13:44:13">]

You can see that simply invoking build_language_grade caused the existing model object in the database to be "nullified". According to the documentation, build_association should simply create a model instance in memory... the database should be unaffected, and the database object should definitely not be nullified.