Trying not to let ActiveRecord leak out...

thing_controller.rb

# This is pretty typical, and often 'advised' in Rails apps today
class ThingController < ApplicationController

  def create
    @thing = current_user.things.build(params[:thing]) # YUCK!
    if @thing.save
      redirect_to(user_thing_path(@thing.seller, @thing))
    else
      render :new
    end
  end

end

thing_controller_2.rb

# I think this might be a better abstraction
class ThingController < ApplicationController

  def create
    seller = Seller.new(current_user)
    @thing = seller.list_thing(params[:thing])
    if @thing.valid?
      redirect_to(user_thing_path(@thing.seller, @thing))
    else
      render :new
    end
  end

end

class Seller
  initialize(user)
    @user = user
  end

  def list_thing(thing_attrs)
    @user.things.create(thing_attrs)
  end
end