SQL Query to Rails 3 for average user ratings - sql

Question

I have a rails app that rates restaurants on specific dimensions using the letsrate gem. I'd like to calculate the average of all ratings for each restaurant and display it to the user in an array on the index page.
My SQL query would look like this -
select avg(stars) from RESTAURANTS r, RATES rs
where rs.rateable_id = r.id
group by r.name;
The array in my index looks like -
<% #restaurants.each do |restaurant| %>
<li>
<a href="<%=restaurant_path(restaurant) %>" >
<div class="left">
<h2 class="name"><%= restaurant.name %></h2>
<h3 class="location"><%= restaurant.location %></h3>
</div>
<div class="right">
<h4 class="rate">AVERAGE RATING</h4>
</div>
<div class="clear"></div>
</a>
</li>
<% end %>
Wondering how I would translate the sql query into rails to display the averages in the array.

Answer 1

If you setup your relations correctly, this should work. If not I'll help you fix them.
Edit
In your Restaurant Controller:
class RestaurantController > ApplicationController
def index
#restaurants = Rate.joins(:restaurant).select("avg(rates.stars) as res_avg, restaurants.name, restaurant.location").group("restaurants.name")
end
end
In your Restaurant index.html.erb:
<% #restaurants.each do |restaurant| %>
<li>
<a href="<%=restaurant_path(restaurant.id) %>" >
<div class="left">
<h2 class="name"><%= restaurant.name %></h2>
<h3 class="location"><%= restaurant.location %></h3>
</div>
<div class="right">
<h4 class="rate"><%= restaurant.res_avg %></h4>
</div>
<div class="clear"></div>
</a>
</li>
<% end %>
Edit2
If you want to re-use this query declare it in a scope on the model.
class Restaurant < ActiveRecord::Base
#all your model code
scope :avg_restaurant_rates, joins(:rate).select("avg(rates.stars) as res_avg, restaurants.name, restaurants.location").group("restaurants.name")
end