Saving belongs_to associations

I'm trying to save a username along with profiles (User has_many Profiles; Profile belongs_to User). I had a portion of this working (updating user info) but I cannot create new user records. I get the following error:

You have a nil object when you didn't expect it! The error occurred while evaluating nil.build

Parameters: {"commit"=>"Create record", "profile"=>{"company"=>"", "new_user_attributes"=>[{"username"=>"jlee", "active"=>"1"}], "new_someid_attributes"=>[{"someid"=>"123456789"}], "type_id"=>"2", "first_name"=>"Jeffrey", "last_name"=>"Lee"}, "authenticity_token"=>"65507986d1912380edf522b5b9f7ee3763c6dc0d"}

I am getting stuck on the profile model code below:

  # Users   def new_user_attributes=(user_attributes)     user_attributes.each do |attributes|       user.build(attributes)     end   end   def existing_user_attributes=(user_attributes)     user.reject(&:new_record?).each do |user|       attributes = user_attributes[user.id.to_s]       if attribute         user.attributes = attributes       else         user.delete(email)       end     end   end   def save_user     user.save(false)   end

New method in profiles_controller:   def new     @profile = Profile.new     @profile.user = User.new   end

User partial: <div class="user">   <% new_or_existing = user.new_record? ? 'new' : 'existing' %>   <% prefix = "profile[#{new_or_existing}_user_attributes]" %>   <% fields_for prefix, user do |user_form| -%>     <%= label :user_form, :user, "Username" %><%= user_form.text_field :username %>     <%= user_form.hidden_field :active, :value => "1" %>   <% end -%> </div>