Declare Acronyms in Rails Inflections

Every Rails app generates config/initializers/inflections.rb to let you customise this behaviour. Most developers leave it empty. Then one day you namespace a controller under API and Rails starts generating Api::UsersController instead of API::UsersController.

Instead of…

…accepting the wrong casing in your class names:

# app/controllers/api/users_controller.rb
class Api::UsersController < ApplicationController
end

Use…

…inflect.acronym to teach Rails the correct casing:

# config/initializers/inflections.rb
ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.acronym "API"
end

Now Rails expects API::UsersController. The file path stays lowercase (app/controllers/api/), but the class name uses the acronym:

# app/controllers/api/users_controller.rb
class API::UsersController < ApplicationController
end

Why?

The acronym method tells ActiveSupport’s inflector to preserve the casing you specify. It affects camelize, underscore, classify, and titleize—which means it also affects autoloading and URL helpers.

"api".camelize        #=> "API"
"API".underscore      #=> "api"
"api/users".camelize  #=> "API::Users"

Without the acronym declaration, you get Api instead:

"api".camelize  #=> "Api"

Unlike irregular plurals and uncountable words, Rails ships with no built-in acronyms—every one you need, you have to declare yourself. Common ones worth adding: API, SMS, CSV, HTML, PDF. You need one call per term:

ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.acronym "API"
  inflect.acronym "SMS"
  inflect.acronym "CSV"
end

This also works for mixed-case words like GraphQL or GitHub. inflect.acronym "GraphQL" ensures "graphql".camelize returns "GraphQL" rather than "Graphql".

See the ActiveSupport::Inflector::Inflections documentation for details.

Note that because these changes are in an initializer, you’ll need to restart your Rails server after making changes.

Why not?

Keep the list short. Every entry changes how camelize, titleize, humanize, and underscore behave for the specified words across your entire app. Only add acronyms you’re actively using—whether in class names, attribute labels, or view helpers.

This only affects casing, not pluralisation. For words with non-standard plurals like criterion/criteria, you’ll want inflect.irregular. For words that don’t pluralise at all, the method to look at is inflect.uncountable.