Defining nested Ruby constants

Basics

Just so that we have something to build on, let’s start with normal constants in Ruby.

They are usually defined in upper case with underscores.

class Account
  STATUS_ACTIVE = 'active'
  STATUS_BANNED = 'banned'
end

Problems with getting all constants

The most common way of having all of them available, is to just define it, like this:

class Account
  STATUS_ACTIVE = 'active'
  STATUS_BANNED = 'banned'

  ALL_STATUSES = [STATUS_ACTIVE, STATUS_BANNED]
end

In th past, I needed it for:

• generating API documentation and showing all statuses Account::ALL_STATUSES.to_sentence => "active and banned"
• generating <select> HTML element
• validation validates :status, inclusion: { in: ALL_STATUSES } (more in docs)

The main downside of this approach is that you have to remember to include it in ALL_STATUSES. For simpler cases is not a problem, but it gets hairy for more complicated cases, such as:

class Request
  SUCCESS = 200
  CREATED = 201
  CLIENT_ERROR = 400
  NOT_FOUND = 404
  SERVER_ERROR = 500

  SUCCESS_CODES = [SUCCESS, CREATED]
  CLIENT_ERROR_CODES = [CLIENT_ERROR, NOT_FOUND]
  SERVER_ERRORS = [SERVER_ERROR]

  # More common approach
  ALL_CODES = [SUCCESS, CREATED, CLIENT_ERROR, NOT_FOUND, SERVER_ERROR]

  # Little bit smarter approach
  ALL_CODES = SUCCESS_CODES + CLIENT_ERROR_CODES + SERVER_ERRORS
end

Now when you add a new code, you’ll need to update the right success/client errors/server errors hash and potentially even ALL_CODES.

DISCLAIMER: Please do not roll you own HTTP client, this is just an example. In my previous jobs we dealt with payment statuses, loan application statuses, fraud statuses where it isn’t that obvious how they relate to each other. Hence, we were more likely to make and a mistake, and unfortunately we made it a few times.

The trick

The core of the “trick” is to understand return value of the constant definition is just the value you assigned:

2.7.2 : 001 > CONSTANT = 1
=> 1

This allows us to write the Request like this:

class Request
  ALL_CODES = [
    SUCCESS_CODES = [
      SUCCESS = 200,
      CREATED = 201,
    ],
    CLIENT_ERROR_CODES = [
      CLIENT_ERROR = 400,
      NOT_FOUND = 404,
    ],
    SERVER_ERRORS = [
      SERVER_ERROR = 500
    ],
  ].flatten
end

Nicer git diff

When introducing new constant, with the smarter approach, the git diff is super obvious:

class Request
  ALL_CODES = [
    SUCCESS_CODES = [
      SUCCESS = 200,
      CREATED = 201,
    ],
    CLIENT_ERROR_CODES = [
      CLIENT_ERROR = 400,
+     FORBIDDEN = 403,
      NOT_FOUND = 404,
    ],
    SERVER_ERRORS = [
      SERVER_ERROR = 500
    ],
  ].flatten
end

Compared to “flattened” approach you have to update multiple places and as a reviewer, it is a bit trickier to know what’s changing

class Request
  SUCCESS = 200
  CREATED = 201
  CLIENT_ERROR = 400
+ FORBIDDEN = 403
  NOT_FOUND = 404
  SERVER_ERROR = 500

  SUCCESS_CODES = [SUCCESS, CREATED]
- CLIENT_ERROR_CODES = [CLIENT_ERROR, NOT_FOUND]
+ CLIENT_ERROR_CODES = [CLIENT_ERROR, FORBIDDEN, NOT_FOUND]
  SERVER_ERRORS = [SERVER_ERROR]

  # More common approach
- ALL_CODES = [SUCCESS, CREATED, CLIENT_ERROR, NOT_FOUND, SERVER_ERROR]
+ ALL_CODES = [SUCCESS, CREATED, CLIENT_ERROR, FORBIDDEN, NOT_FOUND, SERVER_ERROR]

  # Little bit smarter approach
  ALL_CODES = SUCCESS_CODES + CLIENT_ERROR_CODES + SERVER_ERRORS
end

Conclusion

This obvious, yet simple trick shows the structure via nesting, is less error prone, git diff is clearer, and uses just plain Ruby. What’s not to love?

Got questions, comments, or feedback? Reach out at:

Did you know you can define #Ruby constants in a nested way?

Visit https://t.co/RV0yKUDhlG for a deeper explanation with examples pic.twitter.com/4mlv3KkkSr

— Miroslav Csonka (@miroslavcsonka) April 1, 2021

Liked what you just read? Subscribe for new posts