Answer a question

ActionView::Template::Error (PG::Error: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

I'm creating an events website and I'm trying to sort the rendered rsvps by the start time of the event. There are a lot of RSVPS so I'm grouping them with distinct, but I've been having a lot of difficulty over the last few days on sorting the results without this error popping up on PG. I've looked at some of the previous questions on the topic and am still pretty lost. How can I get this to work? Thank you so much!

@rsvps = Rsvp.where(:voter_id => current_user.following.collect {|f| f["id"]}, :status => 'going').where("start_time > ? AND start_time < ?", Time.now, Time.now + 1.month).order("count_all desc").count(:group => :event_id).collect { |f| f[0] }

<%= render :partial => 'rsvps/rsvp', :collection => Rsvp.where(:event_id => @rsvps).select("DISTINCT(event_id)").order('start_time asc') %>

Answers

The ORDER BY clause can only be applied after the DISTINCT has been applied. Since only the fields in the SELECT statement are taken into consideration for the DISTINCT operations, those are the only fields may be used in the ORDER BY.

Logically, if you just want a distinct list of event_id values, what order they occur in should be irrelevant. If order does matter, then you should add the start_time to the SELECT list so that there is context for the order.

Also, these two SELECT clauses are NOT equivalent, so be careful:

SELECT DISTINCT(event_id, start_time) FROM ...

SELECT DISTINCT event_id, start_time FROM ...

The second is the form you want. The first will return a series of records with the data represented as a ROW construct (a single column with a tuple inside). The second will return normal columns of data output. It only works as expected in the single-column case where the ROW construct is reduced down since it is only a single column.