_form.html.erb

<%= form_for(lead) do |f| %>
<%= f.label :email %>
<%= f.text_field :email %>
<%= f.fields_for :referral do |builder| %>
  <%= builder.hidden_field :user_id, :value => "2" %>
<% end %>
<%= f.submit %>
<% end %>

Lead.rb

class Lead < ActiveRecord::Base
  attr_accessible :first_name, :last_name, :email, :phone, :address, :city, :state, :zipcode, :referral_attributes
  has_one :referral
  has_one :user, :through => :referral
  delegate :name, :first_name, :last_name, :email, :phone, :address, :city, :state, :zip, :to => :user, :prefix =>true
  # self.per_page = 5
  validates_presence_of :email
  validates_uniqueness_of :email
  accepts_nested_attributes_for :referral


end

leads_controller.rb

def new
    #@lead = Lead.new
    #@user = @
    @user = User.find_by_slug(request.subdomain)
    # @name1 = user.name
    @lead = Lead.new
    @lead.build_referral
    if !user_signed_in?
	  respond_to do |format|
        format.html # new.html.erb
        format.json { render json: @lead }
      end
    end

      if user_signed_in?
        redirect_to "/webinar"
      end
	end
	def create
      email = params[:lead][:email]
      @lead = Lead.find_by_email(email)
      if @lead 
        session[:user_id] = @lead.id
        session[:login_type] = "lead"
        redirect_to root_url, notice: 'logged in'
         
      else
        @lead = Lead.create(params[:lead])
        # @lead = @lead.build_referral(user_id: 2)
        @lead.save
         respond_to do |format|
          if @lead.save
            format.html { redirect_to @lead, notice: 'User was successfully created.' }
            format.json { render json: @lead, status: :created, location: @lead }
          else
            format.html { render "new" }
            format.json { render json: @lead.errors, status: :unprocessable_entity }
          end
        end
      end
    end