def update_positions(context_ids)
                context_ids.each_with_index {|id, position|
                  context = self.detect { |c| c.id == id.to_i }
                  raise I18n.t('models.user.error_context_not_associated', :context => id, :user => @user.id) if context.nil?
                  context.update_attribute(:position, position + 1)