#questa funziona
def self.all_by_popularity
    return Trip.find(:all,:include => :trip_users, :group => :trip_id,:order => "COUNT(trip_id) DESC")
end



#questa invece restituisce

# ActiveRecord::StatementInvalid in Home#index

# Showing home/index.html.erb where line #45 raised:

# Mysql::Error: #42S22Unknown column 'trip_id' in 'order clause': SELECT id FROM `trips`  GROUP BY trip_id ORDER BY
# COUNT(trip_id) DESC LIMIT 10

def self.all_by_popularity
    return Trip.find(:all,:include => :trip_users, :group => :trip_id,:order => "COUNT(trip_id) DESC",:limit => 10)
end