```ruby
class Collection < ActiveRecord::Base
  belongs_to :user
  has_many :book_collections, :dependent => :destroy
  has_many :books, :through => :book_collections
  has_many :comments, :dependent => :destroy
  validates :name, :presence => true, :uniqueness => true

  scope :most_recent, -> { order(created_at: :desc).limit(1) }
  scope :most_books, -> { select('collections.*, count(books.id) as book_count').
    joins(:books).
    group('collections.id').
    order('book_count DESC LIMIT 1') }

  def books_attributes=(books_attributes)
    books_attributes.values.each do |book_attr|
      book = Book.find_or_create_by(title: book_attr[:title], author: book_attr[:author], publisher: book_attr[:publisher], published_date: book_attr[:published_date], categories: book_attr[:categories], description: book_attr[:description], price: book_attr[:price], isbn: book_attr[:isbn], buy_link: book_attr[:buy_link], image_link: book_attr[:image_link])

      self.book_collections.build(book_id: book.id)
    end
  end
end
```

Remove category from book attributes
user
```ruby
class Collection < ActiveRecord::Base
  belongs_to :user
  has_many :book_collections, :dependent => :destroy
  has_many :books, :through => :book_collections
  has_many :comments, :dependent => :destroy
  validates :name, :presence => true, :uniqueness => true

  scope :most_recent, -> { order(created_at: :desc).limit(1) }
  scope :most_books, -> { select('collections.*, count(books.id) as book_count').
    joins(:books).
    group('collections.id').
    order('book_count DESC LIMIT 1') }

  def books_attributes=(books_attributes)
    books_attributes.values.each do |book_attr|
      book = Book.find_or_create_by(title: book_attr[:title], author: book_attr[:author], publisher: book_attr[:publisher], published_date: book_attr[:published_date], description: book_attr[:description], price: book_attr[:price], isbn: book_attr[:isbn], buy_link: book_attr[:buy_link], image_link: book_attr[:image_link])

      self.book_collections.build(book_id: book.id)
    end
  end
end
```
assistant