Omniauth facebook sessions controller create action integration

I have just finished the Michael Hartl tutorial. I am trying to implement omniauth-facebook to sign-up and sign-in users. I am having trouble creating the master variable to use in the create action in the sessions controller. My assumption is that I should put in an if else statement to see if the master is either signing on through facebook or through the default form? Or should I use and or statement:

(master = Master.find_by(email: params[:session][:email].downcase) || Master.from_omniauth(env["omniauth.auth"])) ?

Here is the omniauth-facebook sessons controller create action code:

def create     user = User.from_omniauth(env["omniauth.auth"])     session[:user_id] = user.id     redirect_to root_url   end

And here is my current sessions controller create action code:

class SessionsController < ApplicationController

    def new     end

    def create         master = Master.find_by(email: params[:session][:email].downcase)         if master && master.authenticate(params[:session][:password])             sign_in master             redirect_back_or master         else             flash.now[:error] = 'Invalid email/password combination'             render 'new'         end     end     .     .     .

end