Multi Select: attaches an additional empty id - ruby-on-rails

Question

when I want to create a new book, I need to attach it to categories. but when I try to save a new book, I get the error: "Could not find the category with < id "=". > I do not know why, but one more id attach besides those I have already attached. And this id is empty ("").
it looks like this ("book" => {"name" => "12", "category_ids" => ["", "2", "7"], ....). Help me please.
my bookcontroler:
def new
#book = Book.new(book_params)
end
def create
#book = Book.new(book_params)
#category = Category.find(id: params[:ids])
BookCategory.create!(book: #book, category: #category)
#book.save
end
private
def book_params
params.require(:book).permit(:name, :author, :image, :description)
end
my form:
<%= form_for (#book) do |f| %>
<%= f.collection_select :category_ids, Category.order(:name), :id, :name, {},{ multiple: :true } %><br>
<% end %>
my error: enter image description here

Answer 1

Provided you have setup the associations correctly:
class Book < ApplicationRecord
has_many :book_categories
has_many :categories, through: :book_categories
end
class Category < ApplicationRecord
has_many :book_categories
has_many :books, through: :book_categories
end
class BookCategory < ApplicationRecord
belongs_to :book
belongs_to :category
end
For every has_many association ActiveRecord creates an _ids setter and getter:
#book = Book.find(1)
#book.category_ids
# => [1, 2, 3]
#book.category_ids = [2,4] # will add 4 and remove 1 and 3.
So you don't need to manually setup the associations, just pass the category_ids param:
def new
#book = Book.new # don't assign params in the new method!
end
def create
#book = Book.new(book_params)
if #book.save
redirect_to #book
else
render :new
end
end
private
def book_params
# category_ids: [] allows an array of scalar values
params.require(:book)
.permit(:name, :author, :image, :description, category_ids: [])
end
Your code won't work since you are using .find with an array param that can be nil or empty.
Also the options argument for collection_select is optional:
<%= form_for (#book) do |f| %>
<%= f.collection_select :category_ids, Category.order(:name), :id, :name, { multiple: :true } %><br>
<% end %>