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"