discourse/app/models/user_search.rb

class UserSearch

  def self.search term, topic_id = nil
    User.find_by_sql sql(term, topic_id)
  end

  private

  def self.sql term, topic_id
    sql = "select id, username, name, email from users u "
    if topic_id
      sql << "left join (select distinct p.user_id from posts p where topic_id = :topic_id) s on
        s.user_id = u.id "
    end

    if term.present?
      sql << "where username ilike :term_like or
              to_tsvector('simple', name) @@
              to_tsquery('simple',
                regexp_replace(
                  regexp_replace(
                    cast(plainto_tsquery(:term) as text)
                    ,'\''(?: |$)', ':*''', 'g'),
                '''', '', 'g')
              ) "

    end

    sql << "order by case when username_lower = :term then 0 else 1 end asc, "
    if topic_id
      sql << " case when s.user_id is null then 0 else 1 end desc, "
    end

    sql << " case when last_seen_at is null then 0 else 1 end desc, last_seen_at desc, username asc limit(20)"

    sanitize_sql_array(sql, topic_id: topic_id, term_like: "#{term}%", term: term)
  end

  def self.sanitize_sql_array *args
    ActiveRecord::Base.send(:sanitize_sql_array, args)
  end

end