Topic: Model return false and display flash

So I have couple custom class methods in my models

  def self.request(user, friend, date, message)
    user_id = user.id
    friend_id = friend.id

    unless date.nil? || user_id == friend_id || exists?(user_id, friend_id)
      transaction do
        user.meetings.create!(friend_id: friend_id, status: 'pending', when: date, message: message)
        friend.meetings.create!(friend_id: user_id, status: 'requested', when: date, message: message)
      end
    else
      false
    end
  end

And here's the connecting controller

  def create
    friend = User.where(id: params[:friend]).first
    time = Chronic.parse(params[:when]) # Parse the input date

    if Meeting.request(current_user, friend, time, params[:message])
      redirect_to :back, notice: "You have requested to hangout with #{friend}."
    else
      redirect_to :back, error: "There was an error sending the hangout request to #{friend}, please try again."
    end
  end

So here were creating a new appointment, and when the request fails it doesn't show the flash message

redirect_to :back, error: "There was an error sending the hangout request to #{friend}, please try again."

But if the appointment is created it does show the notice flash message, What do I have to do in the class method to make the error flash render??