Order by association created_at, or by own created_at if no association

问题

My group model has_many posts, posts have_many comments.

When I show the posts on a group, I want to order by the created at of the most recent comment, if there are any on a post, or by the created_at of the post itself.

So an old post would jump back to the top of the list if it has a new comment.

回答1:

I would stick to a little different approach: your Comment belongs_to :post and belongs_to accepts :touch option. Set it to true and your Post's updated_at will be automatically updated on a Comment's modification. This way you can freely show your posts ordered by their updated_at.

回答2:

You can add commented_at:datetime column into posts table, then add before_create callback to Post model:

before_create :set_default_commented_at

def set_default_commented_at
  self.commented_at ||= Time.now
end

And add :touch to belongs_to association of Comment model:

belongs_to :post, touch: :commented_at

After all you'll be able to order by commented_at column.

来源：https://stackoverflow.com/questions/13604931/order-by-association-created-at-or-by-own-created-at-if-no-association