ruby-on-rails - rails 4 : Want to check if an object already exists when submitting a form and then update that record with additional info (many-to-many relationship)

Question

我正在构建一个在线个人图书馆应用程序，我希望拥有它以便用户可以将书籍添加到他们的图书馆。我想检查并查看要添加到图书馆的图书是否已存在于数据库中，然后将该图书的实例添加到该图书馆中。现在，我的表单设置为始终创建一本书的新实例，并且不检查该书是否存在。我在 Rails 方面还很陌生，如果有任何帮助，我将不胜感激。这是我的代码:

型号

图书馆.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

  validates :user_id, presence: true
  validates :name, presence: true, length: { maximum: 100 }
  ...
end

书.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

  validates :title, presence: true, length: { maximum: 140 }
  validates :author, presence: true, length: { maximum: 140 }
  validates :publisher, presence: true, length: { maximum: 140 }
  validates :isbn, presence: true, uniqueness: true
  ...
end

目录.rb:

class Catalog < ActiveRecord::Base
  belongs_to :book
  belongs_to :library
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, shelf_ids: [])
  end
  ...
end

查看

_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.collection_select :shelf_ids, current_user.library.shelves.all, :id, :name, {:selected => @book.shelf_ids, :include_blank => true}, {:multiple => true} %>
  </div>
  <%= f.submit "Add Book to Library", class: "btn btn-primary" %>
<% end %>

Answer 1

这听起来很适合 find_or_initialize_by .

在那种情况下，您将在您的模型中执行此操作。因此，如果它找到一个存在的实例，它将返回该实例，如果不存在，它将返回一个新实例(在您的 Controller 中):

编辑，您可能需要手动传递您的 book 参数(以避免表单中的其他参数)。还编辑以反射(reflect)货架。

 @book = Book.find_or_initialize_by(name: params[:name], author:  params[:author], publisher: params[:publisher], isbn: params[:isbn]])
 shelfs = Shelf.find(params[:shelf_ids]) 
 @book.shelfs = shelfs
 @book.save

当涉及到shelfs 时，可能还有许多其他方法可以工作，但这会根据参数找到它们的所有实例，然后将它们分配给这本书 实例。