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

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):

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.