how to save nested form attributes to database - ruby-on-rails

Question

I do not really understand how nested attributes work in Rails.
I have 2 models, Accounts and Users. Account has_many Users. When a new user fills in the form, Rails reports
User(#2164802740) expected, got Array(#2148376200)
Can Rails not read the nested attributes from the form? How can I fix it? How can I save the data from nested attributes to the database?
Thanks all~
Here are the MVCs:
Account Model
class Account < ActiveRecord::Base
has_many :users
accepts_nested_attributes_for :users
validates_presence_of :company_name, :message => "companyname is required."
validates_presence_of :company_website, :message => "website is required."
end
User Model
class User < ActiveRecord::Base
belongs_to :account
validates_presence_of :user_name, :message => "username too short."
validates_presence_of :password, :message => "password too short."
end
Account Controller
class AccountController < ApplicationController
def new
end
def created
end
def create
#account = Account.new(params[:account])
if #account.save
redirect_to :action => "created"
else
flash[:notice] = "error!!!"
render :action => "new"
end
end
end
Account/new View
<h1>Account#new</h1>
<% form_for :account, :url => { :action => "create" } do |f| %>
<% f.fields_for :users do |ff| %>
<p>
<%= ff.label :user_name %><br />
<%= ff.text_field :user_name %>
</p>
<p>
<%= ff.label :password %><br />
<%= ff.password_field :password %>
</p>
<% end %>
<p>
<%= f.label :company_name %><br />
<%= f.text_field :company_name %>
</p>
<p>
<%= f.label :company_website %><br />
<%= f.text_field :company_website %>
</p>
<% end %>
Account Migration
class CreateAccounts < ActiveRecord::Migration
def self.up
create_table :accounts do |t|
t.string :company_name
t.string :company_website
t.timestamps
end
end
def self.down
drop_table :accounts
end
end
User Migration
class CreateUsers < ActiveRecord::Migration
def self.up
create_table :users do |t|
t.string :user_name
t.string :password
t.integer :account_id
t.timestamps
end
end
def self.down
drop_table :users
end
end
Thanks everyone. :)

Answer 1

Change the following view region:
<% form_for :account, :url => { :action => "create" } do |f| %>
into:
<% form_for #account do |f| %>
Inside your controller you should have something like this:
def new
#account = Account.new
# the new empty account doesn't have any users
# so the user fields inside your view won't appear unless you specify otherwise:
#account.users.build
#account.users.build
#account.users.build
end