Rails: Checkboxes for Many-to-Many Relationship with hidden fields

I'm building an application (web/iOS) that allows a user to set a series of preference options. The models/tables required include:

• Users: stores user name, password, email address


• Prefs: stores names/classes of preferences available, i.e., "sex"


• PrefOpts: stores options for each preference, i.e., "Male" and "Female" for sex


• UserPrefs: stores a selected PrefOpt for each User for each Pref

Model Specs:

class User < ActiveRecord::Base
  has_many :user_prefs
  has_many :prefopts, through: :user_prefs

end

class Pref < ActiveRecord::Base
  has_many :prefopts
  has_many :user_prefs

  accepts_nested_attributes_for :prefopts

  validates :name, presence: true
end

class Prefopt < ActiveRecord::Base
  belongs_to :pref
  has_many :user_prefs
  has_many :users, through: :user_prefs

  accepts_nested_attributes_for :user_prefs
end

class UserPref < ActiveRecord::Base
  belongs_to :user
  belongs_to :prefopt
end

For now, I want to set the user's preferences/options on the user "show" page, so when I pull up a user's record, I see a listing of all the preferences and for each a drop-down list of the available preferences for each option.

I have updated the Users controller to query back the preferences...

  # GET /users/1
  # GET /users/1.json
  def show
    @prefs = Pref.all
  end

Also, I added to the routes file references under users:

  resources :users do
    resources :prefs do
      get 'prefopts', on: :member
    end
  end

And this works fine: on a user's "show" page I can see all the available preferences when using this syntax:

<p>
  <H2>Preferences</H2>
  <ul>
    <% @prefs.each do |pref| %>
    <li><%= pref.name %></li>
      <ul>
      </ul>

    <% end %>
  </ul>
</p>

Now, I want to create a series of check-boxes with each option available for each preference. From other examples, I've put together this:

<%= form_for(@user) do |f| %>
      <%= f.hidden_field :user_id, :value => @user.id %>
  <H2>Preferences</H2>
  <ul>
    <% @prefs.each do |pref| %>
    <li><%= pref.name %></li>
      <%= f.fields_for :user_prefs, @user.user_prefs.find_or_initialize_by(pref_id: pref.id) do |u| -%>
      <%= u.hidden_field :pref_id, pref.id %>
      <%= u.collection_check_boxes(:user_pref, :pref_opt_ids, pref.prefopts, :id, :name) %>
      <% end -%>

    <% end %>
  </ul>
</p>

The error I'm getting is on the line with find_or_initialize_by and is: undefined method `merge' for 1:Fixnum

I'd greatly appreciate any assistance you can give me!