OK, so the user can now click on the "Movie Title" or "Release Date"

headings and see movies sorted by those columns, and can additionally

use the checkboxes to restrict the listing to movies with certain

ratings only. And we have preserved RESTfulness, because the URI itself

always contains the parameters that will control sorting and filtering.

The last step is to remember these settings. That is, if the user has

selected any combination of column sorting and restrict-by-rating

constraints, and then the user clicks to see the details of one of the

movies (for example), when she clicks the Back to Movie List on the

detail page, the movie listing should "remember" her sorting and

filtering settings from before.

(Clicking away from the list to see the details of a movie is only one

example; the settings should be remembered regardless what actions the

user takes, so that any time she visits the index page, the settings are

correctly reinstated.)

The best way to do the "remembering" will be to use the `session[]`

hash. The session is like the `flash[]`, except that once you set

something in the `session[]` it is remembered "forever" until you nuke the

session with `session.clear` or selectively delete things from it with

`session.delete(:some_key)`. That way, in the `index` method, you can

selectively apply the settings from the `session[]` even if the incoming

URI doesn’t have the appropriate `params[]` set.

If the user explicitly includes new sorting/filtering settings in

`params[]`, the session should not override them. Instead, these new

settings should be remembered in the session.

If a user unchecks all checkboxes, use the settings stored in the

`session[]` hash, since it doesn't make sense for a user to uncheck all the

boxes.

To be RESTful, we want to preserve the property that a URI that results

in a sorted/filtered view always contains the corresponding

sorting/filtering parameters. Therefore, if you find that the incoming

URI is lacking the right `params[]` and you're forced to fill them in from

the `session[]`, the RESTful thing to do is to `redirect_to` the new URI

containing the appropriate parameters. There is an important corner case

to keep in mind here, though: if the previous action had placed a

message in the `flash[]` to display after a redirect to the movies page,

your additional redirect will delete that message and it will never

appear, since the `flash[]` only survives across a single redirect. To fix

this, use `flash.keep` right before your additional redirect.

Deploy to Heroku by following the same process as before:

$ git commit -a -m "part 3 complete"

$ git push heroku master