How to get the checked boxes with link_to into params?

Question

I have a Rails 5.2.1 app with Movies and Categories. They are connected to one another through a has_and_belongs_to_many relation with a join table.

Trying to do the following: on the index page for Movies, I want to filter the collection of movies that is shown by checking Category check boxes. I can properly show check boxes for the Categories, but I'm having a hard time getting the information about what check boxes are checked into the params.

/ rails_app/app/views/movies/index.html.slim
h1 Listing movies

= collection_check_boxes(@movies, :category_ids, Category.all, :id, :name) do |box|
  = box.check_box
  = box.label

= link_to 'Filter movies on category', params.permit!.merge(filter: :category_ids)

table
  / table with movies that will be filtered

These :category_ids seem to be wrong. Is it possible to somehow get at the check box results in this way (for further filtering with query string parameters)? Am I missing something, e.g. in my controller?

# rails_app/app/controllers/movies_controller.rb
class MoviesController < ApplicationController
  def index
    @movies = Movie.all
  end

...

  def movie_params
    params.require(:movie).permit(:name, :rating, category_ids: [])
  end
end

The above is an example app, generated with some scaffolds and a few edits:

rails generate scaffold category name:string
rails generate scaffold movie name:string rating:integer
rails generate migration CreateJoinTableMoviesCategories movie category
bin/rails db:migrate RAILS_ENV=development
-> add has_and_belongs_to_many :movies to Category class
-> add has_and_belongs_to_many :categories to Movie class
-> add category_ids: [] to movie_params in Movie class

Answer 1

try this and see if it works for you.

View:

    / rails_app/app/views/movies/index.html.slim

    = form_for :movie do |f|
     = f.collection_check_boxes(:category_ids, Category.all, :id, :name) do |box|
      = box.check_box
      = box.label
     = f.submit

    / table with movies that will be filtered

Controller:

# rails_app/app/controllers/movies_controller.rb
class MoviesController < ApplicationController
  def index
    @movies = if params[:movie]
                Movie.joins(:categories).where(categories: { id: params[:movie][:category_ids] })               
              else
                Movie.all
              end
  end
  ... 

Essentially, wrapping the checkboxes inside a form, then tweak index action when filter params exists.

Note: I'm not familiar with slim syntax, so tweak it if you get syntax error :).