_form.html.erb

<%= form_for([@ticketable, @ticket]) do |f| %>

routes.rb

resources :user do
    resources :tickets
end

ticket.rb

class Ticket < ActiveRecord::Base
  belongs_to :ticketable, :polymorphic => true
end

ticket_controller.rb

class TicketsController < ApplicationController

  before_filter :get_object, :only => [:create, :new]

  def create
    @ticket = Ticket.new(params[:ticket])
    @ticket.ticketable = @ticketable

    respond_to do |format|
      if @ticket.save
        format.html { redirect_to(@ticket, :notice => 'Ticket was successfully created.') }
        format.xml  { render :xml => @ticket, :status => :created, :location => @ticket }
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @ticket.errors, :status => :unprocessable_entity }
      end
    end
  end

  #...other controller code

  private
  def get_object
    type = ''
    if params[:user_id]; type = 'user'
    elsif params[:g_server_id]; type = 'g_server'
    end
    value = type.classify.constantize.find_by_identifier(params[:"#{type}_id"])
    instance_variable_set("@ticketable" , value)
  end
end

user.rb

# part of the user model
class User < ActiveRecord::Base
  has_many :tickets, :as => :ticketable
end