Rails: How can I access the parent model of a new record's nested associations?

Suppose we have the standard Post & Comment models, with Post having accepts_nested_attributes_for :commments and :autosave => true set.

We can create a new post together with some new comments, e.g.:

@post = Post.new :subject => 'foo'
@post.comments.build :text => 'bar'
@post.comments.first # returns the new comment 'bar'
@post.comments.first.post # returns nil :(
@post.save # saves both post and comments simultaneously, in a transaction etc
@post.comments.first # returns the comment 'bar'
@post.comments.first.post # returns the post 'foo'

However, I need to be able to distinguish from within Comment (e.g. from its before_save or validation functions) between

1. this comment is not attached to a post (which is invalid)
2. this comment is attached to an 开发者_Go百科unsaved post (which is valid)

Unfortunately, merely calling self.post from Comment doesn't work, because per above, it returns nil until after save happens. In a callback of course, I don't (and shouldn't) have access to @post, only to self of the comment in question.

So: how can I access the parent model of a new record's nested associations, from the perspective of that nested association model?

(FWIW, the actual sample I'm using this with allows people to create a naked "comment" and will then automatically create a "post" to contain it if there isn't one already. I've simplified this example so it's not specific to my code in irrelevant ways.)

I think it is strange that Rails does not let you do this. It also affects validations in the child model.

There's a ticket with much discussion and no resolution in the Rails bug tracker about this:

Nested attributes validations circular dependency

And a proposed resolution:

nested models: build should directly assign the parent

Basically, the deal is, the nested attributes code doesn't set the parent association in the child record.

There's some work-arounds mentioned in the second ticket I linked to.

I don't think you can do this. On the other hand, your validations shouldn't be failing, as the order of the transaction will create the post record before saving the comment.