Five Things to Avoid in Ruby

Martin Streicher

Martin Streicher on

Five Things to Avoid in Ruby

As a contract software developer, I am exposed to oodles of Ruby code. Some code is readable, some obfuscated. Some code eschews whitespace, as if carriage returns were a scarce natural resource, while other code resembles a living room fashioned by Vincent Van Duysen. Code, like the people who author it, varies.

Yet, it's ideal to minimize variation. Time and effort are best spent on novel problems.

In this post, we'll explore five faux pas often seen in Ruby code and learn how to turn these idiosyncrasies into idioms.

But first, how can we minimize variance in Ruby?

Minimizing Variance in Ruby with Rubocop and Idioms

Ruby developers can gain efficiency with Rubocop, which unifies style within a single project or across many projects. Rails is a boon to uniformity, too, as it favors convention over configuration. Indeed, disparate Rails codebases are identical in places and, overall, quite similar.

Beyond tools and convention, variance is also minimized by idioms, or those "structural forms peculiar to a language". For example, Ruby syntax is a collection of explicit idioms enforced by the interpreter. Reopening a class is a Ruby idiom, too.

But not all Ruby idioms are dicta. Most Ruby idioms are best practices, shorthand, and common usages Ruby developers share informally and in practice.

Learning idioms in Ruby is like learning idioms in any second spoken (or programming) language. It takes time and practice. Yet the more adept you are at recognizing and proffering Ruby's idioms, the better your code will be.

What to Avoid in Ruby

Now, let's move on to looking at five things to avoid in Ruby:

  • Verbosity
  • Long expressions to detect nil
  • Overuse of self
  • Collecting results in temporary variables
  • Sorting and filtering in memory


If you've delved into Ruby, you can appreciate how expressive, compact, fun, and flexible it is. Any given problem can be solved in a number of ways. For example, if/unless, case, and the ternary operator ?: all express decisions — which one you should apply depends on the problem.

However, per Ruby idiom, some if statements are better than others. For instance, the following blocks of code achieve the same result, but one is idiomatic to Ruby:

# Verbose approach actor = nil if response == 1 actor = "Groucho" elsif response == 2 actor = "Chico" elsif response == 3 actor = "Harpo" else actor = "Zeppo" end
# Idiomatic approach actor = if response == 1 "Groucho" elsif response == 2 "Chico" elsif response == 3 "Harpo" else "Zeppo" end

Almost every Ruby statement and expression yields a value, including if, which returns a final code statement value and a matching condition. The version of the if statement in the second code block above leverages this behavior. If response is 2, actor is set to Chico. Assigning the result of an if statement is idiomatic to Ruby. (The same construct can be applied to case, unless, begin/end, and others.)

Another Ruby idiom is present: you need not predefine a variable used within an if statement (or while, for, and others). So, the latter code removes the line actor = nil. In Ruby, unlike other languages, the body of an if statement is not considered a separate scope.

Long Expressions to Detect nil

nil represents "nothing" in Ruby. It's a legitimate value and is its own class (NilClass) with methods. Like other classes, if you call a method that's not defined on nil, Ruby throws an exception akin to undefined method 'xxx' for nil:NilClass.

To avoid this exception, you must first test a variable to determine if it's nil. For example, code such as this is common in Rails applications:

if user && user.plan && == 'Standard' # ... some code for the Standard plan end

The trouble is that such a long chain of assertions is unwieldy. Imagine having to repeat the condition user && user.plan && ... every time you have to reference a user's plan name.

Instead, use Ruby's safe navigation operator, &. It is shorthand for the logic "If a method is called on nil, return nil; otherwise, call the method per normal." Using &. reduces the code above to the much more readable:

if user&.plan&.name == 'Standard' // ... some code end

If user is nil and plan is nil, the expression user&.plan&.name is nil.

The example above assumes user and plan represent a custom structure or Rails model of some kind. You can also use the safe navigation operator if a value represents an Array or Hash.

For example, assume a_list_of_values is an Array:


If the variable a_list_of_values is nil, an exception is thrown. If you expectantly try a_list_of_values&.[index], a syntax error occurs. Instead, use &. with the Array#at method.


Now, if a_list_of_values is nil, the result of the expression is nil.

Overuse of self

Ruby uses self in three substantive ways:

  1. To define class methods
  2. To refer to the current object
  3. To differentiate between a local variable and a method if both have the same name

Here's an example class demonstrating all three usages.

class Rectangle def self.area(length, width) new(length, width).area end def self.volume(length, width, height) area(length, width) * height end def initialize(length, width, height = nil) self.length = length self.width = width self.height = height end def area length * width end def volume area = 100 self.area * height end private attr_reader :length, :width, :height end

def self.area is an example of the first purpose for self, defining a class method. Given this class, the Ruby code puts Rectangle.area(10, 5) produces 50.

The code self.length = length demonstrates the second application of self: the instance variable length is set to the value of the argument length. (The attr_reader statement at the bottom defines the instance variable and provides a getter method.) Here, the statement self.length = length is functionally the same as @length = length.

The third use of self is shown in the volume method. What does puts Rectangle.volume(10, 5, 2) emit? The answer is 100. The line area = 100 sets a method-scoped, local variable named area. However, the line self.area refers to the method area. Hence, the answer is 10 * 5 * 2 = 100.

Consider one more example. What is the output if the volume method is written like this?:

def volume height = 10 self.area * height end

The answer is 500 because both uses of height refer to the local variable, not the instance variable.

Inexperienced Rails developers make unnecessary use of self, as in:

# Class User # first_name: String # last_name: String # ... class User < ApplicationRecord ... def full_name "#{self.first_name} #{self.last_name}" end end

Technically, this code is correct, yet using self to refer to model attributes is unnecessary. If you combine Rubocop with your development environment, Rubocop flags this issue for you to correct.

Collecting Results in Temporary Variables

A common code chore is processing lists of records. You might eliminate records due to certain criteria, map one set of values to another, or separate one set of records into multiple categories. A typical solution is to iterate over the list and accumulate a result.

For instance, consider this a solution to find all even numbers from a list of integers:

def even_numbers(list) even_numbers = [] list.each do |number| even_numbers << number if number&.even? end return even_numbers end

The code creates an empty list to aggregate results and then iterates over the list, ignoring nil items, and accumulating the even values. Finally, it returns the list to the caller. This code serves its purpose, but it isn't idiomatic.

A better approach is to use Ruby's Enumerable methods.

def even_numbers(list) # shorthand for `.select { |v| v.even? }` end

select is one of many Enumerable methods. Each Enumerable method iterates over a list and collects results based on a condition. Specifically, select iterates over a list and accumulates all items where a condition yields a truthy value. In the example above, even? returns true if the item is an even number. select returns a new list, leaving the original list unchanged. A variant named select! performs the same purpose, but alters (mutates) the original list.

The Enumerable methods include reject and map (also known as collect). reject collects all items from a list where a condition yields a falsey value. map returns a new list where each item in the original list is transformed by an expression.

Here's one more example Enumerable method in action. First, some non-idiomatic code:

def transform(hash) new_hash = {} hash.each_pair do |key, value| new_hash[key] = value ? value * 2 : nil end return new_hash end

And now a more idiomatic approach:

def transform(hash) hash.transform_values { |value| value&.*(2) } end

transform_values is another Enumerable method available for Hash. It returns a new hash with the same keys, but each associated value is transformed. Remember, even integers are objects in Ruby and have methods. value&.*(2) returns nil if value is nil, else value * 2.

Sorting and Filtering in Memory

Here's one last faux pas — this one is specific to Rails and ActiveRecord. Let's examine this code:

# class Student # name: String # gpa: Float # ... # class Student < ApplicationRecord ... def self.top_students Student .all .sort_by { |score| student.gpa } .select { |score| student.gpa >= 90.0 } .reverse end end

Calling Student.top_students returns all students with a GPA greater than or equal to 90.0 in ranked order, from highest to lowest. Technically, this code is correct, but it isn't very efficient in space or time because:

  • It must load all records from the students table into memory.
  • It first sorts all records and then filters based on GPA, performing unnecessary work.
  • It reverses the order of the list in memory.

Sorting and filtering are best performed in the database, if possible, using ActiveRecord's tools.

def self.top_students Student.where(gpa: 90.0..).order(gpa: :desc) end

The shorthand 90.0.. in the where clause is a Ruby Range expressing a value between 90.0 and Float::INFINITY. If gpa is indexed in the students table, this query is likely very fast and loads only the matching records. If the student records are being fetched for display, more efficiency (in memory) can be gained via pagination (albeit at the possible expense of more queries to fetch the batches).

Wrapping Up and Next Steps

In this post, we've covered five key things to avoid in Ruby and the idioms to use instead.

I highly recommend reading the documentation for Ruby's core classes and modules, including Array, Hash, and Enumerable. The documents are a treasure trove of methods and techniques, and chances are a method exists to solve your problem at hand. Turn knowledge into practice by writing small code samples to learn how each method works.

Add Rubocop into your workflow, even your text editor. Rubocop keeps your code looking nice but can also flag idiosyncratic code. Using Rubocop is one of the best ways to learn to code the Ruby way.

Finally, read other developers' code, especially open-source Ruby projects. To peruse the code of any gem, run bundle open <gem>, where <gem> is the name of a library. If you've included a debugger in your Gemfile, you can even set breakpoints in any gem and step through the code.

Go forth and code!

P.S. If you'd like to read Ruby Magic posts as soon as they get off the press, subscribe to our Ruby Magic newsletter and never miss a single post!

P.P.S. Use AppSignal for Ruby for deeper debugging insights.

Martin Streicher

Martin Streicher

Our guest author Martin is a professional Ruby developer. He earned an advanced degree in Computer Science from Purdue University, served as Editor-in-Chief of Linux Magazine (US) for five years, and was the founding author of the "Speaking in Unix" column published in IBM's former developerWorks portal. When not coding or writing about code, he collects art and wrangles many small dogs.

All articles by Martin Streicher

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