Ruby on Rails

Accessing a PostgreSQL 9.4.4 database, I have code like:

previous_rsvps = Rsvp
  .select('rsvp.user_id', 'event_sessions.name')
  .where(user_id: selected_attendees).includes(:rsvp_sessions).includes(:event_sessions)
  .where("event_sessions.event_id IS NOT NULL")
  .where("event_sessions.name ~* ? OR event_sessions.name !~* ?",
    "Workshop", "Installfest")

I believe this retrieves the expected records, because if I follow it with a command:
  previous_rsvps.pluck('event_sessions.name').uniq.sort
then the correct names are listed.

However, if I give the command:
  previous_rsvps.count
then I get the error message:
  ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR: missing FROM-clause entry for table "rsvp".

Of course, I'm looking to do more with the previous_rsvps object, but this example captures the error. What causes this error and how do I get around it?