Rails 多对多：通过关联

Question

我是 Rails 的新手，并且已经为此工作了 3-4 天。我已经创建了一个多对多 :through 关联，但是当我提交一个新对象时，我得到这个错误：

#<ActiveModel::Errors:0x007fbced7cda88 @base=#<BlogPost id: nil, created_at: nil, updated_at: nil, title: "title 13", content: "pajfposjpfej 13", posted_by: "poster 13", comments: nil, blog_pic: nil>, @messages={:"categorizations.blog_post"=>["must exist"], :title=>[], :posted_by=>[], :content=>[], :blog_pic=>[]}, @details={"categorizations.blog_post"=>[{:error=>:blank}]}>

我正在提交的表单提交我正在选择一个类别，因此它不应像错误指示的那样为空。非常感谢对此的任何帮助。这几天心灰意冷。

型号：

class BlogPost < ApplicationRecord
    has_many :categorizations
    has_many :categories, :through => :categorizations
    accepts_nested_attributes_for :categorizations
    has_many :comments
    mount_uploader :blog_pic, BlogPicUploader
end

class Categorization < ApplicationRecord
    belongs_to :blog_post
    belongs_to :category
end

class Category < ApplicationRecord
    has_many :categorizations
    has_many :blog_posts, :through => :categorizations 
end

views/blog_posts/new.html.erb

<%= form_for @blog_post do |b| %>
            <%= b.label :title %>
            <%= b.text_field :title %><br>

            <%= b.fields_for :categorizations do |cat| %>
              <%= cat.label :category_name, "Category 1" %>
              <%= cat.collection_select(:category_id, Category.all, :id, :category_name, include_blank: "Select Category") %><br>
            <% end %>

            <%= b.submit "Submit", class: "btn btn-primary" %>
      <% end %>

控制器：

class BlogPostsController < ApplicationController

    def index
      @blog_posts = BlogPost.order(id: :desc)
    end

    def new
      @blog_post = BlogPost.new
      @categorizations = @blog_post.categorizations.build
      @categories = @blog_post.categories.build
    end


...

    def create
      @blog_post = BlogPost.new(blog_post_params)
      respond_to do |format|
          if @blog_post.save
            format.html { redirect_to @blog_post, notice: 'Your blog was submitted successfully' }
            format.json { render :show, status: :created, location: @blog_post }
          else
            format.html { render :new }
            format.json { render json: @blog_post.errors, status: :unprocessable_entity }
          end
        end
        puts @blog_post.errors.inspect
    end

private

  def blog_post_params
    params.require(:blog_post).permit(:title, :content, :posted_by, :comments, :blog_pic, {categorizations_attributes: [:category_id, :category_name]})
  end

end

Answer 1

您还需要为 categories 声明嵌套属性。

喜欢：

class BlogPost < ApplicationRecord
  has_many :categorizations
  has_many :categories, :through => :categorizations
  accepts_nested_attributes_for :categorizations
  accepts_nested_attributes_for :categories
  has_many :comments
  mount_uploader :blog_pic, BlogPicUploader
end

您可以参考下面link的描述

https://hackhands.com/building-has_many-model-relationship-form-cocoon/

Answer 2

这里的答案与代码无关 - 简单地说 - 在 Rails 5 中，他们进行了更改，现在默认需要 belongs_to 关联，而 Rails 4，这是可选的。请参阅下面的 link 解释：

http://blog.bigbinary.com/2016/02/15/rails-5-makes-belong-to-association-required-by-default.html

您必须将 config/initializers/new_framework_defaults 中的这一行从 true 更改为 false:

Rails.application.config.active_record.belongs_to_required_by_default = false

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