Queries on ActiveRecord Association collection object - ruby

Question

I have a set of rows which I've fetched from a table. Let's say the object Rating. After fetching this object, I have say 100 entries from the database.
The Rating object might look like this:
table_ratings
t.integer :num
So what I now want to do is perform some calculations on these 100 rows without performing any other queries. I can do this, running an additional query:
r = Rating.all
good = r.where('num = 2') # this runs an additional query
"#{good}/#{r.length}"
This is a very rough idea of what I want to do, and I have other more complex output to build. Let's imagine I have over 10 different calculations I need to perform on these rows, and so aliasing these in a sql query might not be the best idea.
What would be an efficient way to replace the r.where query above? Is there a Ruby method or a gem which would give me a similar query api into ActiveRecord Association collection objects?

Answer 1

Rating.all returns an array of all Rating objects. From there, shift your focus to selecting and mapping from the array. eg.:
#perfect_ratings = r.select{|x| x.a == 100}
See:
http://www.ruby-doc.org/core-1.9.3/Array.html
ADDITIONAL COMMENTS:
Going over the list of methods available for array, I find myself using the following frequently:
To check a variable against multiple values:
%w[dog cat llama].include? #pet_type # returns true if #pet_type == 'cat'
To create another array(map and collect are aliases):
%w[dog cat llama].map(|pet| pet.capitalize) # ["Dog", "Cat", "Llama"]
To sort and drop duplicates:
%w[dog cat llama dog].sort.uniq # ["cat", "dog", "llama"]
<< to add an element, + to add arrays, flatten to flatten embedded arrays into a single level array, count or length or size for number of elements, and join are the others I tend to use a lot.
Finally, here is an example of join:
%w[dog cat llama].join(' or ') # "dog or cat or llama"