Enterprise Rails

Chapter 11. View-Backed Models

Some concepts are extremely easy to explain in words, yet difficult to extract from a database. For example, on a ticket-purchasing site, it’s pretty obvious that you need a way to quickly show visitors “current movies in my area.” That is certainly an easy concept, but let’s examine what it takes to get that kind of information out of our database.

“Current” means we need to be looking in the movie_showtimes table. We might define current to mean movies starting within a week.

Knowing the showtime isn’t enough. We need to get the movie information, too. A showtime without the movie name isn’t very useful. While we’re at it, we probably need to know the rating and the length of the movie as well.

The request “in my area” means we need to know where the visitor is and where the movie theatre is. We can use the PL/pgSQL distance procedure miles_between_lat_long that we wrote in Chapter 6, but to do so we need data from the theatres table (the zip code) and from the zip_codes table (the latitude and longitude).

This represents a query with four tables: movie_showtimes, theatres, movies, and zip_codes. We could write an ActiveRecord query that could get us what we want:

MovieShowtime.find(:all,
  :include => [:movie, :theatre],
  :conditions => "
        movie_showtimes.start_time - now() < '1 week'::interval
    and movie_showtimes.start_time > now()
  "
)

The first problem we notice, as far as ActiveRecord die-hards are concerned, is that we’ve got ...