以单一形式使用has_many_through和accepts_nested_attributes_for

Question

There are times when Rails makes you laugh with joy, and when it makes you cry with despair. This is one of the latter … Having struggled with this on a more complex app I am building, I have built a simple app that isolates the problem I am having. In summary - a post has a comment and tags. A tag can belong to more than one post, and a post can have more than one tag. I want a nested form that allows me to do this in one swoop. I am using check boxes because they are needed in the more complex app.

comment.rb

class Comment < ApplicationRecord
    belongs_to :post
end

post.rb

class Post < ApplicationRecord
    has_many :comments
    has_many :post_tags
    has_many :tags, through: :post_tags
    accepts_nested_attributes_for :comments
    accepts_nested_attributes_for :tags,  update_only: true
end

post_tag.rb

class PostTag < ApplicationRecord
    belongs_to :post
    belongs_to :tag
end

_form.html.erb

<%= form_with(model: post, local: true) do |form| %>
    <% if post.errors.any? %>
      <div id="error_explanation">
        <h2><%= pluralize(post.errors.count, "error") %> prohibited this post from being saved:</h2>

      <ul>
      <% post.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= form.label :title %>
    <%= form.text_field :title %>
  </div>


  <h2>Comments</h2>
  <%= form.fields_for :comments do |fc| %>
    <%= fc.label :body %>
    <%= fc.text_field :body %>
    <br />
  <% end %>

  <h2>Tags</h2>
  <%= form.fields_for :tags do |ft| %>
    <%= ft.collection_check_boxes(:tag, Tag.all, :id, :tag) do |cb| %>
      <%= cb.label %>
      <%= cb.check_box %>
  <% end %>
    <br />
  <%  end %>


  <div class="actions">
    <%= form.submit %>
  </div>
<% end %>

posts_controller.rb

Standard scaffold - except for this

def new
    @post = Post.new
    @post.comments.build
    @post.tags.build
end


# Never trust parameters from the scary internet, only allow the white list through.
def post_params
    params.require(:post).permit(:title, comments_attributes: [:body, :id], tags_attributes: [{:tag=>[]}, :id])
end

This is the HTML generated by the collection_check_boxes

<input type="hidden" name="post[tags_attributes][0][tag][]" value="" />
    <label for="post_tags_attributes_0_tag_1">tag 1</label>
    <input type="checkbox" value="1" name="post[tags_attributes][0][tag][]" id="post_tags_attributes_0_tag_1" />

    <label for="post_tags_attributes_0_tag_2">tag 2</label>
    <input type="checkbox" value="2" name="post[tags_attributes][0][tag][]" id="post_tags_attributes_0_tag_2" />

    <label for="post_tags_attributes_0_tag_3">tag 3</label>
    <input type="checkbox" value="3" name="post[tags_attributes][0][tag][]" id="post_tags_attributes_0_tag_3" />

    <label for="post_tags_attributes_0_tag_4">tag 4</label>
    <input type="checkbox" value="4" name="post[tags_attributes][0][tag][]" id="post_tags_attributes_0_tag_4" />
<br />
<input type="hidden" value="3" name="post[tags_attributes][0][id]" id="post_tags_attributes_0_id" />

This is what is passed

Parameters: {"utf8"=>"✓", "authenticity_token"=>"0eFWbwi3J6gHnyys6+V/aVymheWE37RWtqq0xm46/umASeyTqj6hTqmTrL4DdvV48yFmMgOtiiYorswTtyCbyw==", 
"post"=>{"title"=>"test post", "comments_attributes"=>{"0"=>{"body"=>"test comment", "id"=>"1"}}, 
"tags_attributes"=>{"0"=>{"tag"=>["", "1", "4"], "id"=>"3"}}}, "commit"=>"Update Post", "id"=>"1"}

The issue lies with the tags_attribute. When the update runs it updates tag entry 3 with the name “["", "1", "4"]” rather than updating the table. I don't know why it is picking up an id of "3". You kind of expect Rails to do everything automagically. When it works it is sweeeeeet. When it doesn't, it is so frustrating. Any advice on where I am going wrong? My suspicion lies with either the collection_check_boxes or the params.require.

Thanks so much.

Answer 1

TL;DR:

• Nested attributes are not needed in your case (so feel free to remove them)

_form.html.erb:

<%= form_with(model: post, local: true) do |form| %>
  <!-- ... -->
  <!-- ... -->
  <%= form.collection_check_boxes(:tag_ids, Tag.all, :id, :tag) do |cb| %>
    <%= cb.label %>
    <%= cb.check_box %>
  <% end %>
<% end %>

posts_controller.rb:

# ...
# ...
def post_params
  params.require(:post).permit(
    :title,
    comments_attributes: [:body, :id],
    tag_ids: []
  )
end

Explanation:

Whenever I think about how to properly set up a form (even if with nested attributes), I usually do it in rails console first. So check out the cool trick below, and you'll immediately get what I mean :)

# rails console
post = Post.first

# hmm I wonder what should we put inside the .update() below, for us to be able to set the associated tags?
post.update(...)

# ok, should be easy enough, let's try below?
post.update(tags: [Tag.find(1), Tag.find(3)])

# cool, above works right?, except... that we cannot use it because we are simulating `params` values,
# ... and `params` values do not have record Objects! but only primitives such as `String` and `Integer`

# hmm... so... why not use the primary keys, right? Let's try below
post.update(tag_ids: [1, 3])

cool, finally it worked! :) basically, then now we just need to somehow translate tag_ids: [1, 3] above, into the form as input fields of which then we should expect to have input fields' name like below (note that TL;DR above already should look like this)

<input type='checkbox' name='post[tag_ids][]' value='1' checked>
<input type='checkbox' name='post[tag_ids][]' value='2'>
<input type='checkbox' name='post[tag_ids][]' value='3' checked>
<input type='checkbox' name='post[tag_ids][]' value='4'>
<input type='checkbox' name='post[tag_ids][]' value='9'>
<!-- OTHER REMAINING TAG IDS HERE. ETC... -->