On Mon, Feb 7, 2011 at 2:28 PM, Rainer Frey <frey.rai...@gmail.com> wrote:
> But this thread seems to suggest one should simply validate the
> association attribute instead. Is that not sufficient then?
You can't still be sure the association is valid, because the
associated object is cached if previously fetched, and the FK can be
changed directly:
fxn@halmos:~/tmp/test_belongs_to ∵ cat app/models/post.rb
class Post < ActiveRecord::Base
has_many :comments
end
fxn@halmos:~/tmp/test_belongs_to ∵ cat app/models/comment.rb
class Comment < ActiveRecord::Base
belongs_to :post
validates :post, :presence => true
end
fxn@halmos:~/tmp/test_belongs_to ∵ cat bypass_validation.rb
post = Post.create
comment = post.comments.create
comment.post_id = -1
p comment.save
comment.reload
p comment.post_id
fxn@halmos:~/tmp/test_belongs_to ∵ rails runner bypass_validation.rb
true
-1
You're going to store the post_id in the database anyway. So if you're
going to take the risk of having dangling records, in my view it's
better to take the risk on the post_id rather than on the association.
I believe that's what the quote from the guide tries to say.
The validates_existence plugin performs a query. That's closer to
checking the association holds an existing record, but there's still
subject to race conditions (say, a concurrent request deleting the
associated record outside your transaction). The only way to be
totally sure the association does exist is to move the check to who
has the key to guarantee that, which is the database with FK
constraints.
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