Teach Rails Irregular Plurals with Inflections

Instead of…

…accepting Rails’s best guess at a plural:

"criterion".pluralize  #=> "criterions"
"matrix".pluralize     #=> "matrices"  # this one Rails knows!

Use…

…inflect.irregular to teach Rails the correct pair:

# config/initializers/inflections.rb
ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.irregular "criterion", "criteria"
end

Now:

"criterion".pluralize  #=> "criteria"
"criteria".singularize #=> "criterion"

Why?

Give irregular the singular and plural forms and Rails handles both directions—pluralize and singularize both work correctly.

A Criterion model will look for a criteria table. resources :criteria will route to CriteriaController. Association names, fixtures, and factory names all follow suit.

class Criterion < ApplicationRecord
  # table: criteria
end

class Survey < ApplicationRecord
  has_many :criteria  # works as expected
end

You can declare as many as you need:

ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.irregular "criterion", "criteria"
  inflect.irregular "goose", "geese"
end

Although, unless you’re building some kind of flighted animal tracker, you probably won’t need that second one.

Rails already knows a handful of irregular plurals: person/people, man/men, child/children, sex/sexes, move/moves, and—crucially—zombie/zombies are built in. Rails’s pluralisation rules are regex-based, so the (m)an → (m)en pattern also covers woman/women. But that’s it—words like tooth/teeth, foot/feet, mouse/mice, and goose/geese are not handled by default. Check the default inflections to see what’s already covered.

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

Why not?

Before adding an irregular inflection, check whether Rails already knows the word. Try it in a console:

"person".pluralize  #=> "people"  — already works
"axis".pluralize    #=> "axes"    — already works

If it’s already correct, adding it to your initialiser is just noise.

If the word never appears as a model or resource name, there’s no reason to declare it.

For words that don’t change between singular and plural (like “sheep” or “metadata”), you need inflect.uncountable. For casing issues with acronyms like API or CSV, look at inflect.acronym.