comment = Comment.new

comment.article_id = Article.first.id

c.valid? #returns true

comment.article_id == comment.article.id #returns true

#So far so good

comment.article_id = nil

c.valid? #Still returns true because the article association is not reloaded for the validation

comment.article_id == comment.article.id #returns false. At this point the comment is valid and a save will succeed but the article_id saved in the db will be null. Perhaps the only way to handle this is to override the article_id= method and force Rails to reload the association:

def article_id=(new_id)
result = super(new_id)
article(true)
result
end

Frankly Rails should be handling all for us this anyway.

AMEN!