Question

I am using devise for authentication.
If the email already exists it logs the user in.
If the email does not exist then it creates a new user using those credentials.
there should be only one submit button and a field for email and password.

Answer 1

that is possible for that have to write condition for that in your controller

for example

def create
 @user = @user.find_or_create_by(params[:user])
 ....
 ....
end

or

def create
  @user = User.find(params[:user])
  unless @user.present?
    @user = User.new(params[:user])
    if @user.valid?
      ...
    else 
      ...
   end
 end
end

or # for devise – in registration controller

def create
  @user = User.find_by_email(params[:user][:email])
  if @user.present? 
    sign_in(resource_name, resource)
  else
    if resource.save
      if resource.active_for_authentication?
       ...
       ...
      else
       ...
       ...
      end
    else
      ...
      ...
    end
  end
end