Refactoring with map, pluck and select in Ruby on Rails

If you need to get something out of the database, consider using pluck to avoid creating instances you don’t need.

I am working with an old code base at the moment and do a lot of refactoring. That provides me with something to think about every day.

In this particular code base I see a lot of transformation of database objects in Ruby, but is that effective? Let’s check and see if we can improve anything.


Before venturing off let’s create a test bench to measure performance:

def m(&block) do
    100.times { yield }

And compare pluck with map on this particular piece of code:

m { stock_movement.positions.pluck(:product_id) }
=> 265.032
m { }
=> 220.282

We see almost no difference, so what’s the point? But performance depends on other factors.

=> 1

Let’s try it with something more substantial:

=> 400
m { }
=> 5647.272

m { stock_movement.positions.pluck(:product_id) }
=> 349.746

Now, we’re talking, but in fact we’re not. This code is used once in the controller, and it might be that there is no actual performance benefit especially when using small amount of data:

m { @stock_movement_positions.pluck(:product_id); }
=> 2367.685
m {; }
=> 367.844

You have to be especially careful inside of the loops. This is not a rule of the thumb but when dealing with database I would conside using pluck. Or select.

select is a different beast. Affected by Rails Magic it can act differently things depending on its position in ActiveRecord chain. Plus there are different flavors of select in ActiveRecord::Association::CollectionProxy, ActiveRecord::QueryMethods etc.

What is important to rememeber is that select might return instanciated real-only objects with paritally filled attributes. It can also return a sub-query, but that’s another topic. Armed with this knowlegde, let’s find a better specimen for refactoring.

A better example

Here is a good candidate for improvement I stumbled upon in a few places. It returns an array of first letters in names in collection to build and alphabetic lister later.'LEFT(name, 1) as letter').group(:name)

What immediately grabs attention is how database output that ends at .all being treated in Ruby. Let’s look at select first:'LEFT(name, 1) as letter').to_sql
=> "SELECT LEFT(name, 1) as letter FROM `stock_products` "

The query does not look particulary mean or anything, again, especially if your database is not that big. Unfortunately for us select instantiates a bunch of half-filled (or half-empty) objects:

=> [#<StockProduct >, #<StockProduct >, #<StockProduct >, ...

This is unacceptable, lets rewrite this using pluck instead of select, DISTINCT instead of uniq and throw away the rest:

StockProduct.pluck('DISTINCT LEFT(name, 1)')

Another thing to keep in mind when using pluck - it returns the array and breaks the ActiveRecord query method chain. And if you need to use the values in a subsequent query you might be actually needing select as it would construct a subquery to use. But in our case we don’t need anything else so that’s fine.

Let’s compare:

m { StockProduct.pluck('DISTINCT LEFT(name, 1)') }
=> 926.424
m {'LEFT(name, 1) as letter').group(:name) }
=> 28469.013

Finally, something good comes out of this refactoring. Moving the code to concern:

# app/models/concerns/search/first_letter.rb
class Search
  module FirstLetter
    extend ActiveSupport::Concern
    included do
      scope :by_letter, -> (letter) { where('name LIKE ?', "#{letter}%") }

    module ClassMethods
      def first_letters
        pluck('DISTINCT LEFT(name, 1)')

Hey, what about sorting?

Okay, you got me. Yes that is a good question. But let’s think about it first. As I output the results in alphabetical order and numbers separated from letters that means I have a knowledge of that.

And this knowledge is kept in the view layer. I am simply intersecting arrays like this:

# views/letters/_letters.html.slim
(?0..?9).to_a & first_letters
(?A..?Z).to_a & first_letters

This construct gives me intersections ordered by alphabet and in numerical order if there are any.

One thing less to worry about. That frees up some brain capacity what is in my book ultimately is the point of refactoring.


Nick Ostrovsky

Full-stack designer. I like everything minimailitic and simple as possible.

Started with Web and Graphics, moved to User Experience and Interfaces, then to Team Lead and CTO. Contributed to projects for major brands such as L'Oreal, JTI and AB InBev among others. Currently working as an independent contractor.