Rails 4: Being able to select multiple instances of one model when creating the model it has a has_many :through relationship with

When I add a new Book to my Library, I want to be able to specify what Shelves in that Library it should belong to. Right now, I'm able to add a Book to my Library, but I'm having issues with the Shelves. I'm pretty new at learning Rails and I'd appreciate any help.

Models

user.rb:

class User < ActiveRecord::Base
  has_one :library, dependent: :destroy
  ...
end

library.rb:

class Library < ActiveRecord::Base
  belongs_to :user
  has_many :shelves, dependent: :destroy

  has_many :catalogs, dependent: :destroy
  has_many :books, :through => :catalogs, dependent: :destroy
  ...
end

book.rb:

class Book < ActiveRecord::Base
  has_many :catalogs, dependent: :destroy
  has_many :libraries, :through => :catalogs, dependent: :destroy

  has_many :bookshelves, dependent: :destroy
  has_many :shelves, :through => :bookshelves, dependent: :destroy
  ...
end

catalog.rb:

class Catalog < ActiveRecord::Base
  belongs_to :book
  belongs_to :library
end

shelf.rb:

class Shelf < ActiveRecord::Base
  belongs_to :library
  has_many :bookshelves, dependent: :destroy
  has_many :books, :through => :bookshelves, dependent: :destroy
  ...
end

bookshelf.rb

class Bookshelf < ActiveRecord::Base
  belongs_to :book
  belongs_to :shelf
end

Controller

books_controller.rb

class BooksController < ApplicationController
  ...

  def create
    @book = Book.new(book_params)
    @library = current_user.library

    if @book.save
      @book.catalogs.create(:library_id => @library.id)
      flash[:success] = "Book added to library!"
      redirect_to current_user
    else
      render 'current_user'
    end
  end

  ...

  private
    def book_params
      params.require(:book).permit(:title, :author, :publisher, :isbn)
    end

    ...
end

View

_book_form.html.erb:

<%= form_for(@book) do |f| %>
  <%= render 'shared/error_messages', object: f.object %>
  <div class="field">
    <%= f.text_field :title, placeholder: "Book Title" %>
    <%= f.text_field :author, placeholder: "Author" %>
    <%= f.text_field :publisher, placeholder: "Publisher" %>
    <%= f.text_field :isbn, placeholder: "ISBN" %>

    <%= f.fields_for :catalogs do |ff| %>
      <%= ff.hidden_field :library_id %>
    <% end %>

    <%= f.fields_for :bookshelves do |ff| %>
      <%= ff.collection_select :shelf_ids, current_user.library.shelves.all.collect {|x| [x.name, x.id]}, {}, multiple: true %>
    <% end %>
  </div>
  <%= f.submit "Add Book to Library", class: "btn btn-primary" %>
<% end %>