class Book
  has_many :book_categories, dependent: :destroy
  has_many :categories, through: :book_categories
end

class BookCategory
  belongs_to :book
  belongs_to :category
end

class Category
  has_many :book_categories
  has_many :books, through: :book_categories
end

def create
  @book = Book.save!(book_params)
end

private

def book_params
  params.require(:book).permit(
    :title,
    category_ids: []
  )
end