Question

Is it possible to write an ActiveRecord query that sorts by an association field if the association exists, and otherwise sorts by an attribute on the object itself?

Example: I have a Discussion object which has_many :comments. I’d like to display a list of discussions sorted by discussion.latest_comment.created_at. However, some discussions may not have any comments, in which case I would like use their discussion.created_at attribute instead.

The catch is that I need the result to be an ActiveRecord::Relation (for performance reasons, and also because we are using Kaminari [which requires a Relation object]).

The only thing I could think of is to actually create a new field like discussion.latest_comment_at which would be initially populated by discussion.created_at and then updated every time a new comment is posted. However, this doesn’t seem very straight-forward from a maintenance perspective (e.g. what happens when a comment gets deleted?).

Answer 1

I don’t know of a way to do this through SQL, so I cheated and I have my code set a last_post_at attribute whenever a topic is created.

That way, I can then sort the topics by last_post_at rather than having to query two tables at once.

I’ve seen other forum systems do it this way too, and it seems like what you’re designing is exactly a forum-like system.