ruby on rails - Must exist error when saving nested attributes through many to many

Question

Trying to save nested records through many-to-many association in Rails 6, but getting "tag must exist" error. Tag is a parent to post_tags which is the cross reference table between Posts and Tags (many-to-many). What I want to do is, when a new post is created, save post_tag records related to the selected tags on the post form. I looked at some related posts: here and here, and tried using inverse_of, autosave: true, and optional: true, but those don't seem to be working.

Here's what I have:

Models

class Post < ApplicationRecord
  has_many :post_tags, dependent: :destroy, inverse_of: :post, autosave: true
  has_many :tags, through: :post_tags
end

class PostTag < ApplicationRecord
  belongs_to :post
  belongs_to :tag
end

class Tag < ApplicationRecord
  has_many :post_tags, dependent: :destroy, inverse_of: :tag, autosave: true
  has_many :posts, through: :post_tags
end

Contoller

PostsController < ApplicationController
  def new
    @post = Post.new
    @tags= Tag.all
    @post.post_tags.build
  end

  def create
    @post = Post.new(post_params)
    @post.post_tags.build
    
    if @post.save
      ...
    end
  end
  
  private

  def post_params
        params.require(:post).permit(:title, :content, :user_id, post_tags_attributes: [tag_id: []])
  end
end

Form

<%= f.fields_for :post_tags do |builder| %>
    <%= builder.collection_check_boxes :tag_id, Tag.top_used, :id, :name, include_hidden: false %>
<% end %>

Error

   (0.4ms)  ROLLBACK
  ? app/controllers/posts_controller.rb:229:in `create'
Completed 422 Unprocessable Entity in 41ms (ActiveRecord: 3.7ms | Allocations: 15178)


  
ActiveRecord::RecordInvalid (Validation failed: Post tags tag must exist):

question from:https://stackoverflow.com/questions/65831401/must-exist-error-when-saving-nested-attributes-through-many-to-many

Answer 1

You don't need to explitly create the "join model" instances. You just need to pass an array to the tag_ids= setter created by has_many :tags, through: :post_tags.

<%= form_with(model: @post) %>
  ...
  <div class="field">
    <%= f.label :tag_ids %>
    <%= f.collection_check_boxes :tag_ids, @tags, :id, :name %>
  </div>
  ...
<% end %>

Your controller should look like:

PostsController < ApplicationController
  def new
    @post = Post.new
    @tags = Tag.all
  end

  def create
    @post = Post.new(post_params)
    if @post.save
      redirect_to @post, status: :created
    else
      @tags = Tag.all
      render :new
    end
  end
  
  private

  def post_params
        params.require(:post)
              .permit(:title, :content, :user_id, tag_ids: [])
  end
end

Using nested attributes and fields_for to create the join model instances is really only needed if you need to store additional information in the join model.