Rails calculate and display average

Question

I'm trying to average scores within a particularly category. I have a table that contains a list of directors, movies, genres, ratings and scores. When the user clicks on a movie I'd like them to be able to see the average score for all other movies in that genre.

I think that I have to use the average method in conjunction with a condition. That said I haven't been able to find any useful documentation online regarding this specific topic. If someone could point me in the direction of a guide or article that explains how to calculate averages with conditions and display them in a view, I would greatly appreciate it!

Maybe http://stackoverflow.com/questions/984995/ruby-on-rails-field-average — Agis, Mar 18 '13 at 20:42

Andrea Fiore · Accepted Answer · 2013-03-18T21:10:33.727

6

Together with other calculations, ActiveRecord provides a method called average. You can use it by implementing something like this:

class Movie
  def self.average_by_genre(genre)
    average(:rating).where(:genre => genre)
  end
end

Alternatively, you can also implement it as an instance method of the Movie model.

class Movie
  def average_rating
    self.class.average(:rating).where(:genre => self.genre)
  end
end

This would let you call the method directly from your view.

<article>
  <header>
    <h1><%= @movie.title %></h1>
  </header>
  <p>
    <span>Average rating for genre <%= @movie.genre %>:</span> 
    <%= @movie.average_rating  %>
  </p>
</article>

edited Mar 18 '13 at 21:10

answered Mar 18 '13 at 20:46

Andrea Fiore

1,628
2
14
18

This is great, thanks! How do I go about displaying this average within the view? – bork121 Mar 18 '13 at 21:00
Do you think calculating average should be done in model, view, or decorator? – mc9 Nov 14 '14 at 04:03

score 5 · Answer 2 · answered Mar 18 '13 at 20:46

5

How about some raw SQL?

SELECT AVG(score) FROM movies WHERE category_id = 42

Or in rails:

Movie.where(category_id: 42).average("score")

Read more info here: http://guides.rubyonrails.org/active_record_querying.html#average

answered Mar 18 '13 at 20:46

Ariejan

10,910
6
43
40

score 0 · Answer 3 · answered Nov 17 '17 at 04:42

0

How about you move SQL raw query to a database view:

# movies_view
SELECT m.id, AVG(m.score) avg_score, c.category_id
FROM movies m
JOIN category c
ON m.category_id

So you will need to query from the MovieView

MovieView.find(:movie_id).avg_score

answered Nov 17 '17 at 04:42

Thanh Khac

3
4

Rails calculate and display average

3 Answers3

Linked