rails 6中的多个下拉菜单

# app/models/category.rb
class Category < ApplicationRecord
end

# app/models/product.rb
class Product < ApplicationRecord
  belongs_to :category
end

<!-- app/views/products/_form.html.erb -->
<%= form_with(model: product, local: true) do |form| %>
  <% if product.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(product.errors.count, "error") %> prohibited this product from being saved:</h2>
      <ul>
        <% product.errors.full_messages.each do |message| %>
          <li><%= message %></li>
        <% end %>
      </ul>
    </div>
  <% end %>

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

  <div class="field">
    <%= form.label :category_id %>
    <%= form.collection_select(:category_id, Category.all, :id, :name) %>
  </div>

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

# app/controllers/products_controller.rb
class ProductsController < ApplicationController
  before_action :set_product, only: [:show, :edit, :update, :destroy]

  def new
    @product = Product.new
  end

  def create
    @product = Product.new(product_params)
    if @product.save
      redirect_to @product, notice: 'Product was successfully created.'
    else
      render :new
    end
  end

  private

  def set_product
    @product = Product.find(params[:id])
  end

  def product_params
    params.require(:product).permit(:name, :category_id)
  end
end

# app/models/category.rb
class Category < ApplicationRecord
end

# app/models/product.rb
class Product < ApplicationRecord
  belongs_to :category
end

# app/controllers/products_controller.rb
class ProductsController < ApplicationController
  before_action :set_product, only: [:show, :edit, :update, :destroy]

  def new
    @product = Product.new
  end

  def create
    @product = Product.new(product_params)
    if @product.save
      redirect_to @product, notice: 'Product was successfully created.'
    else
      render :new
    end
  end

  private

  def set_product
    @product = Product.find(params[:id])
  end

  def product_params
    params.require(:product).permit(:name, :category_id)
  end
end

<!-- app/views/products/_form.html.erb -->
<%= form_with(model: product, local: true) do |form| %>
  <% if product.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(product.errors.count, "error") %> prohibited this product from being saved:</h2>
      <ul>
        <% product.errors.full_messages.each do |message| %>
          <li><%= message %></li>
        <% end %>
      </ul>
    </div>
  <% end %>

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

  <div class="field">
    <%= form.label :category_id %>
    <%= form.collection_select(:category_id, Category.all, :id, :name) %>
  </div>

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