UserSearch refactor

user_search.rb

class UserSearch
  def self.search(term, topic_id = nil)
    scope = User.scoped 

    if topic_id
      scope = scope.joins("
        LEFT JOIN (
          SELECT DISTINCT(posts.user_id)
          FROM posts
          WHERE topic_id = #{topic_id.to_i}
        ) s ON s.user_id = users.id
      ")

      scope = scope.order('s.user_id IS NULL')
    end

    scope = scope.order('
      last_seen_at IS NULL ASC,
      last_seen_at DESC,
      username ASC
    ')

    scope = scope.where("
      username_lower LIKE :term_like OR
      TO_TSVECTOR('simple', name) @@
      TO_TSQUERY('simple',
        REGEXP_REPLACE(
          REGEXP_REPLACE(
            CAST(PLAINTO_TSQUERY(:term) AS text)
            ,'\''(?: |$)', ':*''', 'g'),
        '''', '', 'g')
    )", term: term, term_like: "#{term.downcase}%")
    
    scope.limit(20)
  end
end

I think @blowmage's fork is an improvement over what I produced. See it here:

https://gist.github.com/blowmage/4755700