undefined method `users' for nil:NilClass - ruby-on-rails-3

Question

I am trying to create an application having domains and users, there are 3 types of users, super admin,domain admins and domain users.All the users(3 types) are in the users table and domains in domain table. At present there are domains and super admin. A super admin can login and create domains and domain users. The domains users should be related to a particular domain(i have a "domain_id" column in my 'users' table).Now when i am trying to create a user under a domain(after selecting a domain), i am getting this error "undefined method `users' for nil:NilClass".
My user controller.
def new
#domain = Domain.find(params[:id])
#user = User.new
#title = "Sign up"
end
def create
#user = #domain.users.build(params[:user])
if #user.save
flash[:success] = "Welcome to My Space!"
redirect_to #user
else
#title = "Sign up"
render 'new'
end
end
My user model is having.
belongs_to :domain
validates :domain_id, :presence => true
and domain model is having.
has_many :users
My new.html.erb form
<%= form_for #user do |f| %>
<%= f.label :first_name %>
<%= f.text_field(:first_name, :size => 20) %>
<%= f.label :last_name %>
<%= f.text_field(:last_name, :size => 20) %>
<%= f.label :email %>
<%= f.text_field(:email, :size => 20) %>
<%= f.label :password %>
<%= f.password_field(:password, :size => 20) %>
<%= f.label :password_confirmation, "Verify Password" %>
<%= f.password_field(:password_confirmation, :size => 20) %>
<%= f.submit "Create" %>
<% end %>

Answer 1

Assuming #domain = Domain.find(params[:id]) in the new method does what it should do:
def new
#domain = Domain.find(params[:id])
#user = #domain.users.build
#title = "Sign up"
end
def create
#user = User.new(params[:user])
if #user.save
flash[:success] = "Welcome to My Space!"
redirect_to #user
else
#title = "Sign up"
render 'new'
end
end
And your view should look like this:
<%= form_for #user do |f| %>
<%= f.hidden_field :domain_id %>
<%= f.label :first_name %>
<%= f.text_field(:first_name, :size => 20) %>
<%= f.label :last_name %>
<%= f.text_field(:last_name, :size => 20) %>
<%= f.label :email %>
<%= f.text_field(:email, :size => 20) %>
<%= f.label :password %>
<%= f.password_field(:password, :size => 20) %>
<%= f.label :password_confirmation, "Verify Password" %>
<%= f.password_field(:password_confirmation, :size => 20) %>
<%= f.submit "Create" %>
<% end %>
Because of the hidden domain_id field in your form, the user will be assigned to the correct domain.