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The Inflector transforms words from singular to plural, class names to table names, modularized class names to ones without, and class names to foreign keys. The default inflections for pluralization, singularization, and uncountable words are kept in inflections.rb.

The Rails core team has stated patches for the inflections library will not be accepted in order to avoid breaking legacy applications which may be relying on errant inflections. If you discover an incorrect inflection and require it for your application or wish to define rules for languages other than English, please correct or add them yourself (explained below).

Instance Public methods
camelize(term, uppercase_first_letter = true)

Converts strings to UpperCamelCase. If the uppercase_first_letter parameter is set to false, then produces lowerCamelCase.

Also converts '/' to '::' which is useful for converting paths to namespaces.

camelize('active_model')                # => "ActiveModel"
camelize('active_model', false)         # => "activeModel"
camelize('active_model/errors')         # => "ActiveModel::Errors"
camelize('active_model/errors', false)  # => "activeModel::Errors"

As a rule of thumb you can think of camelize as the inverse of underscore, though there are cases where that does not hold:

camelize(underscore('SSLError'))        # => "SslError"
# File activesupport/lib/active_support/inflector/methods.rb, line 68
def camelize(term, uppercase_first_letter = true)
  string = term.to_s
  if uppercase_first_letter
    string = string.sub(/^[a-z\d]*/) { |match| inflections.acronyms[match] || match.capitalize }
    string = string.sub(inflections.acronyms_camelize_regex) { |match| match.downcase }
  string.gsub!(/(?:_|(\/))([a-z\d]*)/i) { "#{$1}#{inflections.acronyms[$2] || $2.capitalize}" }
  string.gsub!("/", "::")

Creates a class name from a plural table name like Rails does for table names to models. Note that this returns a string and not a Class (To convert to an actual class follow classify with constantize).

classify('ham_and_eggs') # => "HamAndEgg"
classify('posts')        # => "Post"

Singular names are not handled correctly:

classify('calculus')     # => "Calculus"
# File activesupport/lib/active_support/inflector/methods.rb, line 200
def classify(table_name)
  # strip out any leading schema name
  camelize(singularize(table_name.to_s.sub(/.*\./, "")))

Tries to find a constant with the name specified in the argument string.

constantize('Module')   # => Module
constantize('Foo::Bar') # => Foo::Bar

The name is assumed to be the one of a top-level constant, no matter whether it starts with “::” or not. No lexical context is taken into account:

C = 'outside'
module M
  C = 'inside'
  C                # => 'inside'
  constantize('C') # => 'outside', same as ::C

NameError is raised when the name is not in CamelCase or the constant is unknown.

# File activesupport/lib/active_support/inflector/methods.rb, line 271
def constantize(camel_cased_word)
  names = camel_cased_word.split("::")

  # Trigger a built-in NameError exception including the ill-formed constant in the message.
  Object.const_get(camel_cased_word) if names.empty?

  # Remove the first blank element in case of '::ClassName' notation.
  names.shift if names.size > 1 && names.first.empty?

  names.inject(Object) do |constant, name|
    if constant == Object
      candidate = constant.const_get(name)
      next candidate if constant.const_defined?(name, false)
      next candidate unless Object.const_defined?(name)

      # Go down the ancestors to check if it is owned directly. The check
      # stops when we reach Object or the end of ancestors tree.
      constant = constant.ancestors.inject(constant) do |const, ancestor|
        break const    if ancestor == Object
        break ancestor if ancestor.const_defined?(name, false)

      # owner is in Object, so raise
      constant.const_get(name, false)

Replaces underscores with dashes in the string.

dasherize('puni_puni') # => "puni-puni"
# File activesupport/lib/active_support/inflector/methods.rb, line 208
def dasherize(underscored_word)
  underscored_word.tr("_", "-")

Removes the rightmost segment from the constant expression in the string.

deconstantize('Net::HTTP')   # => "Net"
deconstantize('::Net::HTTP') # => "::Net"
deconstantize('String')      # => ""
deconstantize('::String')    # => ""
deconstantize('')            # => ""

See also demodulize.

# File activesupport/lib/active_support/inflector/methods.rb, line 238
def deconstantize(path)
  path.to_s[0, path.rindex("::") || 0] # implementation based on the one in facets' Module#spacename

Removes the module part from the expression in the string.

demodulize('ActiveSupport::Inflector::Inflections') # => "Inflections"
demodulize('Inflections')                           # => "Inflections"
demodulize('::Inflections')                         # => "Inflections"
demodulize('')                                      # => ""

See also deconstantize.

# File activesupport/lib/active_support/inflector/methods.rb, line 220
def demodulize(path)
  path = path.to_s
  if i = path.rindex("::")
    path[(i + 2)..-1]
foreign_key(class_name, separate_class_name_and_id_with_underscore = true)

Creates a foreign key name from a class name. separate_class_name_and_id_with_underscore sets whether the method should put '_' between the name and 'id'.

foreign_key('Message')        # => "message_id"
foreign_key('Message', false) # => "messageid"
foreign_key('Admin::Post')    # => "post_id"
# File activesupport/lib/active_support/inflector/methods.rb, line 249
def foreign_key(class_name, separate_class_name_and_id_with_underscore = true)
  underscore(demodulize(class_name)) + (separate_class_name_and_id_with_underscore ? "_id" : "id")
humanize(lower_case_and_underscored_word, capitalize: true, keep_id_suffix: false)

Tweaks an attribute name for display to end users.

Specifically, performs these transformations:

  • Applies human inflection rules to the argument.

  • Deletes leading underscores, if any.

  • Removes a “_id” suffix if present.

  • Replaces underscores with spaces, if any.

  • Downcases all words except acronyms.

  • Capitalizes the first word.

The capitalization of the first word can be turned off by setting the :capitalize option to false (default is true).

The trailing '_id' can be kept and capitalized by setting the optional parameter keep_id_suffix to true (default is false).

humanize('employee_salary')                  # => "Employee salary"
humanize('author_id')                        # => "Author"
humanize('author_id', capitalize: false)     # => "author"
humanize('_id')                              # => "Id"
humanize('author_id', keep_id_suffix: true)  # => "Author Id"

If “SSL” was defined to be an acronym:

humanize('ssl_error') # => "SSL error"
# File activesupport/lib/active_support/inflector/methods.rb, line 128
def humanize(lower_case_and_underscored_word, capitalize: true, keep_id_suffix: false)
  result = lower_case_and_underscored_word.to_s.dup

  inflections.humans.each { |(rule, replacement)| break if result.sub!(rule, replacement) }

  result.sub!(/\A_+/, "")
  unless keep_id_suffix
    result.sub!(/_id\z/, "")
  result.tr!("_", " ")

  result.gsub!(/([a-z\d]*)/i) do |match|
    "#{inflections.acronyms[match.downcase] || match.downcase}"

  if capitalize
    result.sub!(/\A\w/) { |match| match.upcase }

inflections(locale = :en)

Yields a singleton instance of Inflector::Inflections so you can specify additional inflector rules. If passed an optional locale, rules for other languages can be specified. If not specified, defaults to :en. Only rules for English are provided.

ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.uncountable 'rails'
# File activesupport/lib/active_support/inflector/inflections.rb, line 249
def inflections(locale = :en)
  if block_given?
    yield Inflections.instance(locale)

Returns the suffix that should be added to a number to denote the position in an ordered sequence such as 1st, 2nd, 3rd, 4th.

ordinal(1)     # => "st"
ordinal(2)     # => "nd"
ordinal(1002)  # => "nd"
ordinal(1003)  # => "rd"
ordinal(-11)   # => "th"
ordinal(-1021) # => "st"
# File activesupport/lib/active_support/inflector/methods.rb, line 344
def ordinal(number)
  I18n.translate("number.nth.ordinals", number: number)

Turns a number into an ordinal string used to denote the position in an ordered sequence such as 1st, 2nd, 3rd, 4th.

ordinalize(1)     # => "1st"
ordinalize(2)     # => "2nd"
ordinalize(1002)  # => "1002nd"
ordinalize(1003)  # => "1003rd"
ordinalize(-11)   # => "-11th"
ordinalize(-1021) # => "-1021st"
# File activesupport/lib/active_support/inflector/methods.rb, line 357
def ordinalize(number)
  I18n.translate("number.nth.ordinalized", number: number)
parameterize(string, separator: "-", preserve_case: false, locale: nil)

Replaces special characters in a string so that it may be used as part of a 'pretty' URL.

parameterize("Donald E. Knuth") # => "donald-e-knuth"
parameterize("^très|Jolie-- ")  # => "tres-jolie"

To use a custom separator, override the separator argument.

parameterize("Donald E. Knuth", separator: '_') # => "donald_e_knuth"
parameterize("^très|Jolie__ ", separator: '_')  # => "tres_jolie"

To preserve the case of the characters in a string, use the preserve_case argument.

parameterize("Donald E. Knuth", preserve_case: true) # => "Donald-E-Knuth"
parameterize("^très|Jolie-- ", preserve_case: true) # => "tres-Jolie"

It preserves dashes and underscores unless they are used as separators:

parameterize("^très|Jolie__ ")                 # => "tres-jolie__"
parameterize("^très|Jolie-- ", separator: "_") # => "tres_jolie--"
parameterize("^très_Jolie-- ", separator: ".") # => "tres_jolie--"

If the optional parameter locale is specified, the word will be parameterized as a word of that language. By default, this parameter is set to nil and it will use the configured <tt>I18n.locale<tt>.

# File activesupport/lib/active_support/inflector/transliterate.rb, line 95
def parameterize(string, separator: "-", preserve_case: false, locale: nil)
  # Replace accented chars with their ASCII equivalents.
  parameterized_string = transliterate(string, locale: locale)

  # Turn unwanted chars into the separator.
  parameterized_string.gsub!(/[^a-z0-9\-_]+/i, separator)

  unless separator.nil? || separator.empty?
    if separator == "-"
      re_duplicate_separator        = /-{2,}/
      re_leading_trailing_separator = /^-|-$/i
      re_sep = Regexp.escape(separator)
      re_duplicate_separator        = /#{re_sep}{2,}/
      re_leading_trailing_separator = /^#{re_sep}|#{re_sep}$/i
    # No more than one of the separator in a row.
    parameterized_string.gsub!(re_duplicate_separator, separator)
    # Remove leading/trailing separator.
    parameterized_string.gsub!(re_leading_trailing_separator, "")

  parameterized_string.downcase! unless preserve_case
pluralize(word, locale = :en)

Returns the plural form of the word in the string.

If passed an optional locale parameter, the word will be pluralized using rules defined for that language. By default, this parameter is set to :en.

pluralize('post')             # => "posts"
pluralize('octopus')          # => "octopi"
pluralize('sheep')            # => "sheep"
pluralize('words')            # => "words"
pluralize('CamelOctopus')     # => "CamelOctopi"
pluralize('ley', :es)         # => "leyes"
# File activesupport/lib/active_support/inflector/methods.rb, line 31
def pluralize(word, locale = :en)
  apply_inflections(word, inflections(locale).plurals, locale)

Tries to find a constant with the name specified in the argument string.

safe_constantize('Module')   # => Module
safe_constantize('Foo::Bar') # => Foo::Bar

The name is assumed to be the one of a top-level constant, no matter whether it starts with “::” or not. No lexical context is taken into account:

C = 'outside'
module M
  C = 'inside'
  C                     # => 'inside'
  safe_constantize('C') # => 'outside', same as ::C

nil is returned when the name is not in CamelCase or the constant (or part of it) is unknown.

safe_constantize('blargle')                  # => nil
safe_constantize('UnknownModule')            # => nil
safe_constantize('UnknownModule::Foo::Bar')  # => nil
# File activesupport/lib/active_support/inflector/methods.rb, line 324
def safe_constantize(camel_cased_word)
rescue NameError => e
  raise if e.name && !(camel_cased_word.to_s.split("::").include?(e.name.to_s) ||
    e.name.to_s == camel_cased_word.to_s)
rescue ArgumentError => e
  raise unless /not missing constant #{const_regexp(camel_cased_word)}!$/.match?(e.message)
rescue LoadError => e
  raise unless /Unable to autoload constant #{const_regexp(camel_cased_word)}/.match?(e.message)
singularize(word, locale = :en)

The reverse of pluralize, returns the singular form of a word in a string.

If passed an optional locale parameter, the word will be singularized using rules defined for that language. By default, this parameter is set to :en.

singularize('posts')            # => "post"
singularize('octopi')           # => "octopus"
singularize('sheep')            # => "sheep"
singularize('word')             # => "word"
singularize('CamelOctopi')      # => "CamelOctopus"
singularize('leyes', :es)       # => "ley"
# File activesupport/lib/active_support/inflector/methods.rb, line 48
def singularize(word, locale = :en)
  apply_inflections(word, inflections(locale).singulars, locale)

Creates the name of a table like Rails does for models to table names. This method uses the pluralize method on the last word in the string.

tableize('RawScaledScorer') # => "raw_scaled_scorers"
tableize('ham_and_egg')     # => "ham_and_eggs"
tableize('fancyCategory')   # => "fancy_categories"
# File activesupport/lib/active_support/inflector/methods.rb, line 186
def tableize(class_name)
titleize(word, keep_id_suffix: false)

Capitalizes all the words and replaces some characters in the string to create a nicer looking title. titleize is meant for creating pretty output. It is not used in the Rails internals.

The trailing '_id','Id'.. can be kept and capitalized by setting the optional parameter keep_id_suffix to true. By default, this parameter is false.

titleize is also aliased as titlecase.

titleize('man from the boondocks')                       # => "Man From The Boondocks"
titleize('x-men: the last stand')                        # => "X Men: The Last Stand"
titleize('TheManWithoutAPast')                           # => "The Man Without A Past"
titleize('raiders_of_the_lost_ark')                      # => "Raiders Of The Lost Ark"
titleize('string_ending_with_id', keep_id_suffix: true)  # => "String Ending With Id"
# File activesupport/lib/active_support/inflector/methods.rb, line 174
def titleize(word, keep_id_suffix: false)
  humanize(underscore(word), keep_id_suffix: keep_id_suffix).gsub(/\b(?<!\w['’`])[a-z]/) do |match|
transliterate(string, replacement = "?", locale: nil)

Replaces non-ASCII characters with an ASCII approximation, or if none exists, a replacement character which defaults to “?”.

# => "AEroskobing"

Default approximations are provided for Western/Latin characters, e.g, “ø”, “ñ”, “é”, “ß”, etc.

This method is I18n aware, so you can set up custom approximations for a locale. This can be useful, for example, to transliterate German's “ü” and “ö” to “ue” and “oe”, or to add support for transliterating Russian to ASCII.

In order to make your custom transliterations available, you must set them as the i18n.transliterate.rule i18n key:

# Store the transliterations in locales/de.yml
      ü: "ue"
      ö: "oe"

# Or set them using Ruby
I18n.backend.store_translations(:de, i18n: {
  transliterate: {
    rule: {
      'ü' => 'ue',
      'ö' => 'oe'

The value for i18n.transliterate.rule can be a simple Hash that maps characters to ASCII approximations as shown above, or, for more complex requirements, a Proc:

I18n.backend.store_translations(:de, i18n: {
  transliterate: {
    rule: ->(string) { MyTransliterator.transliterate(string) }

Now you can have different transliterations for each locale:

transliterate('Jürgen', locale: :en)
# => "Jurgen"

transliterate('Jürgen', locale: :de)
# => "Juergen"
# File activesupport/lib/active_support/inflector/transliterate.rb, line 59
def transliterate(string, replacement = "?", locale: nil)
  raise ArgumentError, "Can only transliterate strings. Received #{string.class.name}" unless string.is_a?(String)

    replacement: replacement,
    locale: locale

Makes an underscored, lowercase form from the expression in the string.

Changes '::' to '/' to convert namespaces to paths.

underscore('ActiveModel')         # => "active_model"
underscore('ActiveModel::Errors') # => "active_model/errors"

As a rule of thumb you can think of underscore as the inverse of camelize, though there are cases where that does not hold:

camelize(underscore('SSLError'))  # => "SslError"
# File activesupport/lib/active_support/inflector/methods.rb, line 91
def underscore(camel_cased_word)
  return camel_cased_word unless /[A-Z-]|::/.match?(camel_cased_word)
  word = camel_cased_word.to_s.gsub("::", "/")
  word.gsub!(inflections.acronyms_underscore_regex) { "#{$1 && '_' }#{$2.downcase}" }
  word.gsub!(/([A-Z\d]+)([A-Z][a-z])/, '\1_\2')
  word.gsub!(/([a-z\d])([A-Z])/, '\1_\2')
  word.tr!("-", "_")

Converts just the first character to uppercase.

upcase_first('what a Lovely Day') # => "What a Lovely Day"
upcase_first('w')                 # => "W"
upcase_first('')                  # => ""
# File activesupport/lib/active_support/inflector/methods.rb, line 155
def upcase_first(string)
  string.length > 0 ? string[0].upcase.concat(string[1..-1]) : ""