Homework 2 - task 4

Enhance RottenPotatoes as follows. At the top of the All Movies listing, add some checkboxes
that allow the user to filter the list to show only movies with certain MPAA ratings:

![](http://i.minus.com/i1eerJPsqTD6e.png)

When the Refresh button is pressed, the list of movies is redisplayed
showing only those movies whose ratings were checked.

This will require a couple of pieces of code. We have provided the code
that generates the checkboxes form, which you can include in the
index.html.haml template, [here on Pastebin](http://pastebin.com/agjb5qBF).
BUT, you have to do a bit of work to use it: our code expects the
variable `@all_ratings` to be an enumerable collection of all possible
values of a movie rating, such as `['G','PG','PG- 13','R']`. The
controller method needs to set up this variable. And since the possible
values of movie ratings are really the responsibility of the Movie
model, it's best if the controller sets this variable by consulting the
Model. Create a class method of Movie that returns an appropriate value
for this collection.

You will also need code that figures out (i) how to figure out which
boxes the user checked and (ii) how to restrict the database query based
on that result.

Regarding (i), try viewing the source of the movie listings with the
checkbox form, and you'll see that the checkboxes have field names like
`ratings[G]`, `ratings[PG]`, etc. This trick will cause Rails to
aggregate the values into a single hash called ratings, whose keys will
be the names of the checked boxes only and whose values will be the
value attribute of the checkbox (which is "1" by default, since we
didn't specify another value when calling the `check_box_tag` helper).
That is, if the user checks the 'G' and 'R' boxes, params will include
as one if its values `:ratings=>{"G"=>"1", "R"=>"1"}`. Check out the
Hash documentation for an easy way to grab just the keys of a hash,
since we don't care about the values in this case.

Regarding (ii), you'll probably end up replacing `Movie.all` with
`Movie.find`, which has various options to help you restrict the
database query.

Two caveats:
* Make sure that you don't break the sorted-column functionality you
  added in part 3b! That is, sorting by column headers should still
work, and if the user then clicksthe "Movie Title" column header to sort
by movie title, the displayed results should both be sorted and be
limited by the Ratings checkboxes.
* If the user checks (say) 'G' and 'PG' and then redisplays the list,
  the checkboxes that were used to filter the output should appear
checked when the list is redisplayed. This will require you to modify
the checkbox form slightly from the version we provided above.
* Don't put code in your views! Set up some kind of instance variable in
  the controller that remembers which ratings were actually used to do
the filtering, and make that variable available to the view so that the
appropriate boxes can be pre-checked when the index view is reloaded.
* Update: Make sure that your form elements have the following ids. The
  submit button for filtering by ratings should have an HTML element id
of `ratings_submit`. Each checkbox should have an HTML element id of
`ratings_#{rating}`, where the interpolated rating should be the rating
itself, such as “PG-13”, “G”. An example of an id for the checkbox for
PG-13 is `ratings_PG-13`.