Update database using 'update_attributes' through 'has_many' - ruby-on-rails

Question

I'm having a problem getting my first app (I'm a total newbie) to save a new associated record. I have two models (users and pictures) with a has_many/belongs_to association. I have set up the userController so that it can create a new picture as below:
def new_picture
#user = User.find(current_user.id)
#picture = #user.pictures.build
end
def create_picture
#user = User.find(params[:id])
#picture = #user.pictures.build(params[:picture])
if #picture.save
flash[:notice] = "Your picture was successfully added."
redirect_to :action => 'show', :id => #user.id
else
render :template => "new_picture"
end
end
and I use
<%= link_to("add picture", :action => 'new_picture', :id => #user.id) if current_user %>
to add a new one. But I'd also like to be able to edit. So I updated the usercontroller with some new code:
def edit_picture
#user = User.find(current_user.id)
#picture = #user.pictures.find(params[:id])
end
# When the user clicks the save button update record
def update_picture
#user = User.find(current_user.id)
#picture = #user.pictures.find(params[:picture])
respond_to do |format|
if #picture.update_attributes(params[:picture])
flash[:notice] = "Your picture was successfully updated."
redirect_to :action => 'show', :id => #user.id
else
render :template => "new_picture"
end
end
end
and added the edit link to show.erb:
<%= link_to("edit picture", :action => 'edit_picture', :id => picture.id) if current_user %>
It loads the edit form fine, with the data all in the right place, but on save all it's doing is giving me the error 'ArgumentError in UsersController#update_picture' with a bunch of Unknown key(s) from my pictures table.
Could somebody explain why? I feel like there is one piece of the jigsaw I haven't quite understood here....
Thanks in advance!
UPDATE: View code is as follows:
<h1>New picture for <%= #user.name %></h1>
<% form_for :picture, #picture, :html => { :multipart => true }, :url => {:action => 'update_picture', :id => #user.id} do |f| %>

Answer 1

Can't seem to see your problem in the view code, however you can do the same thing more elegantly (RESTful) as a nested route. That way you might be able to see the problem more clearly.
config/routes.rb:
resources :users do
member do
resources :pictures
end
end
app/controllers/pictures_controller.rb:
class PicturesController < ApplicationController
before_filter :find_picture, :only => [:edit, :update]
def edit
end
def update
if #picture.update_attributes params[:picture]
flash[:notice] = "Your picture was successfully updated."
redirect_to user_path(current_user)
else
render :edit
end
end
protected
def find_picture
#picture = current_user.pictures.find params[:id]
end
end
app/views/pictures/edit.html.erb:
<%= form_for [current_user, #picture] do |f| %>
<!-- some stuff -->
<% end %>
and to link to your edit form:
<%= link_to_if current_user, 'edit picture',
edit_user_picture_path(:user => current_user, :id => picture) %>

Answer 2

I suggest adding 'accepts_nested_attributes_for :pictures to the user model, and then do
<%= form_for #user do |form| %>
.. user fields
<%= form.fields_for :pictures do |picture_form| %>
.. picture fields
<% end %>
<%= form.submit %>
<% end %>
in the view.
Another option is to create a new controller for the pictures. That may be simpler.