Order and sort_by difference in Ruby on Rails ActiveRecord

Question

I am trying to sort my data according to timestamp field in my controller, note that the timestamp field may be null and may have some value. I wrote the following query.

@item = Item.sort_by(&:item_timestamp).reverse
            .paginate(:page => params[:page], :per_page =>5)

But this gives error when I have items that have time_timestamp field value as NULL, but following query works.

@item = Item.order(:item_timestamp).reverse
            .paginate(:page => params[:page], :per_page => 5)

Can anybody tell the difference between these two queries, and in which condition to use which one?

And I am using order and reverse to get the latest items from the database, Is this the best way or there are other best ways to get the latest data from database in terms of performance?

Answer 1

.sort_by is a Ruby method from Enumerable that is used to sort arrays (or array like objects). Using .sort_by will cause all the records to be loaded from the database into the servers memory, which can lead to serious performance problems (as well as your issue with nil values).

.order is a ActiveRecord method that adds a ORDER BY clause to the SQL select statement. The database will handle sorting the records. This is preferable in 99% of cases.

Answer 2

As it was said before me, .order is quicker, and it's enough in most cases, but sometimes you need sort_by, if you want to sort by value in a relation for example.

If you have a posts table and a view_counters table, where you have the number of views by article, you can't easily sort your posts by total views with .order.

But with sort_by, you can just do:

posts = @user.posts.joins(:view_counter)
@posts = posts.sort_by { |p| p.total_views }

.sort_by going to browse each element, get the relation value, then sort by the value of this relation, just with one code line.

You can further reduce the code with &:[attributeName], for example:

@posts = posts.sort_by(&:total_views)

Also, for your last question about the reverse, you can do this:

Item.order(item_timestamp: :desc)