rails - update_attributes for just part of the model; file uploading - ruby-on-rails

Question

I want to add the ability of a user to have several pictures associated with his / her user account.
I have the following classes:
class User < ActiveRecord::Base
has_many :assets
accepts_nested_attributes_for :assets
end
class Asset < ActiveRecord::Base
belongs_to :assetable, :polymorphic => true
belongs_to :user
end
I want to have a screen that just has the upload image functionality:
def add_profile_picture
#user=User.find(params[:id])
1.times {#user.assets.build}
end
form:
<%= form_for #user do |u| %>
<%= u.fields_for :assets do |asset| %>
<%= asset.file_field :asset %>
<%= asset.text_field :description %><br />
<% end %>
<%=u.submit %>
<% end %>
When I submit, it looks like the id value goes in ok in development.log:
"id"=>"1"
but I get the error:
undefined method `update_attributes' for nil:NilClass
Since I just have the asset fields, is there anything special I need to do? Also, because the belongs_to :user exists, could that be causing problems?
Basically:
asset:
user_id:
assetable_type:
assetable_id:
Any help would be appreciated. Don't do much Rails forms stuff.
thx
edit #1
class UsersController < ApplicationController
def add_profile_picture
#user=User.find(params[:id])
1.times {#user.assets.build}
end
thx

Answer 1

Okay - there are a few problems with your code here. I would highly recommend you read both the Action Controller Overview and the Rails Routing guides to get some more information about this.
In any case, you're getting the error because the form you have there will be trying to use the users#update action in the UsersController.
You've got a couple options. One is to create the necessary routes for the custom action, or you can create a nested resource, and make a form for adding the asset.
In this case, you'd do something like this:
in routes.rb
resources :users do
resources :assets, :only => [:new, :create] # Or any other actions you might want. It's best practise to limit these.
end
Then, in the AssetsController, you can do something similar to this:
def new
#asset = Asset.new
end
def create
#asset = Asset.new(params[:asset])
#asset.user_id = params[:user_id] if params[:user_id]
#asset.save!
end
and your form will look something like this:
<%= form_for #asset do |f| %>
<%= f.file_field :asset %>
<%= f.text_field :description %><br />
<%=f.submit %>
<% end %>