Changing foreign key doesn't set relationship as dirty

I have come across a problem which I believe is closely related to this.

If I have a model that belongs_to another, and requires the presence of the relationship (i.e., a valid non-null foreign key), I can get AR to save invalid records in the database.

For example, with this...

Class Parent < AR::B
end

class Child < AR::B
  belongs_to :parent # using parent_id
  validates_presence_of :parent
end

./script/console...
# child = Child.new
# child.valid? # false
# child.parent_id = Parent.first.id # a valid parent
# child.valid? # true
# child.parent_id = 9999 # doesn't exist
# child.valid? # true
# child.save   # true
# child.reload
# child.parent_id # 9999
# child.parent    # nil
# child.valid? # false