how to create a sign_i function that doesn't require a password in rails code example

Example: how to create a sign_i function that doesn't require a password in rails

def register
  @user = User.new
end

def create
  @user = User.new(user_params)

  if @user.save
    redirect_to root_path, notice: 'Welcome! We have sent you the link to login to our app'
  else
    render :register
  end
end

private

def user_params
  params.require(:user).permit(:fullname, :username, :email)
end