Which approach to use for validating presence of a belongs_to association?

Question

Given

class Post
  has_many :comments
end

class Comment
  belongs_to :post
end

If I want to validate that a comment always has a post, should I use

class Comment
  belongs_to :post
  validates :post, presence: true
end

or

class Comment
  belongs_to :post
  validates :post_id, presence: true
end

?

The Rails guides suggests the validates :post, presence: true approach, saying:

If you want to be sure that an association is present, you'll need to test whether the associated object itself is present, and not the foreign key used to map the association.

One difference I can see between the two is that if you do

post = Post.new
comment = Comment.new
comment.post = post
comment.save

then validation fails if it's based on post_id, but succeeds if it's based on post. The latter makes more sense to me.

However, I sometimes see people in real life using the post_id approach. Are they simply using the "wrong" approach, or is there a rationale I'm not aware of?

The Rails Style Guide doesn't have anything related to this topic.

The question Rails ActiveRecord:: Proper way for validating presence on associations? seems to be asking about how to get a project to work using the post_id approach, but doesn't seem to explain why it's using that approach.

Rails 4: Difference between validates presence on id or association is about validation, but merely describes the different effects of the two approaches, rather than saying why post_id should be used.

The question Rails - Validate Presence Of Association? is unrelated - it's talking about validating that a post has at least one comment, which is a different requirement to what I'm asking about.

score 1 · Answer 1 · answered Oct 28 '15 at 02:57

I don't think a single case would fit all scenarios.

In the case of posts and comments, I would tend to validate on the foreign_key rather than the association. This is partly due to better performance but also testing the Comment model would become independent of the Post model.

For the example of Comment and Post, when creating a comment, I would set association (i.e. comment.post = post or post.comments.create!) rather than the foreign_key (i.e. comment.post_id = post.id), so I know that the post exists (and that's why I am not validating the presence of the association). However in the unlikely case of data corruption, this will not have a significant impact on the application (just one comment wouldn't be associated with an existing post).

In other scenarios, where you need to be absolutely sure that the association is present, you could be extremely safe and validate the association (for instance a CreditCard belongs to a User or an Invoice belonging to an Account).

score 0 · Answer 2 · edited May 23 '17 at 12:03

The comments section of the blog post http://railsguides.net/belongs-to-and-presence-validation-rule1/ , linked to from the answer https://stackoverflow.com/a/25808182/38765 suggests that some people validate on the post_id just because they weren't aware of other ways of doing it:

Thanks for posting this! I've always validated on the field. I didn't even think about validating the association.

and

Amazing, thank you. That was subtle.

One person said post_id may have better performance:

Keep in mind that validating on :account instead of :account_id will incur a performance penalty, as the database has to be queried to check that the association exists.

but I tend to be wary of doing things for performance reasons.

Which approach to use for validating presence of a belongs_to association?

2 Answers2