Доступ к параметру модели через связь many-to-many

  <%= form_for(@item) do |f| %>
 
    <%= f.label :name %>
    <%= f.text_field :name %>
 
    <%= f.label :avatar %>
    <%= f.text_field :avatar %>
 
    <%= f.label :description %>
    <%= f.text_field :description %>
 
    <%=f.label :parent_category %>
    <%= f.select(:id, options_from_collection_for_select(Category.all, :id, :name),
                 {include_blank: 'None'}, {:class => 'form-control'}) %>
 
    <%= f.submit 'Create new item!', class: "btn btn-large btn-primary" %>
<% end %>

 def create
    @item = Item.new(item_params)
    if @item.save
      redirect_to @item
    else
      render '/'
    end
  end

has_many :products_categories, dependent: :destroy
has_many :categories, through: :products_categories
 
has_many :products_categories, dependent: :destroy
has_many :products, through: :products_categories

 def create
    @category = Category.find(params[:id])
    @item = Item.new(item_params)
    @item.categories = @category
    if @item.save
      redirect_to @item
    else
      render '/'
    end
  end

<%= f.select(:categories,
             options_from_collection_for_select(Category.all,
                                                :id,
                                                :name,
                                                @item.categories.map(&:id)),
             {include_blank: 'None'},
             {class: 'form-control', size: 5, multiple: true}) %>

@product.new(product.params)
@product.categories = Category.where(id: params[:product][:categories])
if @product.save
...

class Item < ActiveRecord::Base
  mount_uploader :avatar, AvatarUploader
 
  has_many :item_categories, dependent: :destroy
  has_many :categories, through: :item_categories
 
end

class Category < ActiveRecord::Base
 
    has_many :item_categories, dependent: :destroy
    has_many :items, through: :item_categories
 
end

class ItemCategory < ActiveRecord::Base
  belongs_to :item
  belongs_to :category
end

  <% @items.each do |item| %>
      <li>
        <%= item.name %>
        <%= item.description %>
        <p> Category: </p>
            <% @categories.each do |category| %>
            <% if item[:category_id] == category.id   %>
                <%= category.name %>
            <% end %>
        <% end %>
        <% if item[:category_id].nil? %>
            <p> This item doesn't have category. </p>
        <% end %>
      </li>
      </br>
  <% end %>

...
def index
  @items = Item.includes(:categories).all
end
...

<% @items.each do |item| %>
  <li>
    <%= item.name %>&nbsp;
    <%= item.description %><br>
    <strong> Category: </strong>
    <% if item.categories.any? %>
      <p>
        <% item.categories.each do |category| %>
          <%= link_to category.name, category_path(category) %>&nbsp;
        <% end %>
      </p>
    <% else %>
      <p> This item doesn't have category. </p>
    <% end %>
  </li>
<% end %>