acts.rb

class ActsController < ActionController::Base
  before_filter :get_user

  def new
    @act = @user.acts.new
  end

  def create
    @act = @user.acts.new params[:act]

    respond_to do | format |
      if @act.save
        flash[:notice] = "Act uploaded successdfully"
        format.html { redirect_to @act }
      else
        flash[:error] = "Error occurred. Please try again."
        format.html { render action: 'new' }
      end
    end
  end

private
  def get_user
    @user = User.find session[:user_id]
  end
end

class Act < ActiveRecord::Base
  attr_accessible :file
  attr_accessor :file

  before_validation :ensure_file_is_valid?

protected
  def ensure_file_is_valid?
    if file.nil?
      errors.add(:base, "")
    end
  end
end

# _form.html.erb
<%= form_for([@user, @act]) do | f | %>
  <%= f.file_field :file %>
<% end %>