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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).

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Instance Public methods
apply_inflections(word, rules, locale = :en)

Applies inflection rules for singularize and pluralize.

If passed an optional locale parameter, the uncountables will be found for that locale.

apply_inflections('post', inflections.plurals, :en)    # => "posts"
apply_inflections('posts', inflections.singulars, :en) # => "post"
# File activesupport/lib/active_support/inflector/methods.rb, line 386
    def apply_inflections(word, rules, locale = :en)
      result = word.to_s.dup

      if word.empty? || inflections(locale).uncountables.uncountable?(result)
        result
      else
        rules.each { |(rule, replacement)| break if result.sub!(rule, replacement) }
        result
      end
    end
end
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 69
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 }
  else
    string = string.sub(inflections.acronyms_camelize_regex) { |match| match.downcase }
  end
  string.gsub!(/(?:_|(\/))([a-z\d]*)/i) { "#{$1}#{inflections.acronyms[$2] || $2.capitalize}" }
  string.gsub!("/".freeze, "::".freeze)
  string
end
classify(table_name)

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 201
def classify(table_name)
  # strip out any leading schema name
  camelize(singularize(table_name.to_s.sub(/.*\./, "".freeze)))
end
const_regexp(camel_cased_word)

Mounts a regular expression, returned as a string to ease interpolation, that will match part by part the given constant.

const_regexp("Foo::Bar::Baz") # => "Foo(::Bar(::Baz)?)?"
const_regexp("::")            # => "::"
# File activesupport/lib/active_support/inflector/methods.rb, line 367
def const_regexp(camel_cased_word)
  parts = camel_cased_word.split("::".freeze)

  return Regexp.escape(camel_cased_word) if parts.blank?

  last = parts.pop

  parts.reverse.inject(last) do |acc, part|
    part.empty? ? acc : "#{part}(::#{acc})?"
  end
end
constantize(camel_cased_word)

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
end

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

# File activesupport/lib/active_support/inflector/methods.rb, line 272
def constantize(camel_cased_word)
  names = camel_cased_word.split("::".freeze)

  # 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
      constant.const_get(name)
    else
      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)
        const
      end

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

Replaces underscores with dashes in the string.

dasherize('puni_puni') # => "puni-puni"
# File activesupport/lib/active_support/inflector/methods.rb, line 209
def dasherize(underscored_word)
  underscored_word.tr("_".freeze, "-".freeze)
end
deconstantize(path)

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 239
def deconstantize(path)
  path.to_s[0, path.rindex("::") || 0] # implementation based on the one in facets' Module#spacename
end
demodulize(path)

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 221
def demodulize(path)
  path = path.to_s
  if i = path.rindex("::")
    path[(i + 2)..-1]
  else
    path
  end
end
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 250
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")
end
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 129
    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_+/, "".freeze)
      unless keep_id_suffix
        result.sub!(/_id\z/, "".freeze)
      end
      result.tr!("_".freeze, " ".freeze)

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

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

      result
    end

    # Converts just the first character to uppercase.
    #
    #   upcase_first('what a Lovely Day') # => "What a Lovely Day"
    #   upcase_first('w')                 # => "W"
    #   upcase_first('')                  # => ""
    def upcase_first(string)
      string.length > 0 ? string[0].upcase.concat(string[1..-1]) : ""
    end

    # 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"
    def titleize(word, keep_id_suffix: false)
      humanize(underscore(word), keep_id_suffix: keep_id_suffix).gsub(/\b(?<!\w['’`])[a-z]/) do |match|
        match.capitalize
      end
    end

    # 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"
    def tableize(class_name)
      pluralize(underscore(class_name))
    end

    # 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"
    def classify(table_name)
      # strip out any leading schema name
      camelize(singularize(table_name.to_s.sub(/.*\./, "".freeze)))
    end

    # Replaces underscores with dashes in the string.
    #
    #   dasherize('puni_puni') # => "puni-puni"
    def dasherize(underscored_word)
      underscored_word.tr("_".freeze, "-".freeze)
    end

    # 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.
    def demodulize(path)
      path = path.to_s
      if i = path.rindex("::")
        path[(i + 2)..-1]
      else
        path
      end
    end

    # 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.
    def deconstantize(path)
      path.to_s[0, path.rindex("::") || 0] # implementation based on the one in facets' Module#spacename
    end

    # 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"
    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")
    end

    # 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
    #   end
    #
    # NameError is raised when the name is not in CamelCase or the constant is
    # unknown.
    def constantize(camel_cased_word)
      names = camel_cased_word.split("::".freeze)

      # 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
          constant.const_get(name)
        else
          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)
            const
          end

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

    # 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
    #   end
    #
    # +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
    def safe_constantize(camel_cased_word)
      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)
    end

    # 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"
    def ordinal(number)
      I18n.translate("number.nth.ordinals", number: number)
    end

    # 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"
    def ordinalize(number)
      I18n.translate("number.nth.ordinalized", number: number)
    end

    private

      # Mounts a regular expression, returned as a string to ease interpolation,
      # that will match part by part the given constant.
      #
      #   const_regexp("Foo::Bar::Baz") # => "Foo(::Bar(::Baz)?)?"
      #   const_regexp("::")            # => "::"
      def const_regexp(camel_cased_word)
        parts = camel_cased_word.split("::".freeze)

        return Regexp.escape(camel_cased_word) if parts.blank?

        last = parts.pop

        parts.reverse.inject(last) do |acc, part|
          part.empty? ? acc : "#{part}(::#{acc})?"
        end
      end

      # Applies inflection rules for +singularize+ and +pluralize+.
      #
      # If passed an optional +locale+ parameter, the uncountables will be
      # found for that locale.
      #
      #  apply_inflections('post', inflections.plurals, :en)    # => "posts"
      #  apply_inflections('posts', inflections.singulars, :en) # => "post"
      def apply_inflections(word, rules, locale = :en)
        result = word.to_s.dup

        if word.empty? || inflections(locale).uncountables.uncountable?(result)
          result
        else
          rules.each { |(rule, replacement)| break if result.sub!(rule, replacement) }
          result
        end
      end
  end
end
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'
end
# File activesupport/lib/active_support/inflector/inflections.rb, line 252
def inflections(locale = :en)
  if block_given?
    yield Inflections.instance(locale)
  else
    Inflections.instance(locale)
  end
end
ordinal(number)

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 343
def ordinal(number)
  I18n.translate("number.nth.ordinals", number: number)
end
ordinalize(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 356
def ordinalize(number)
  I18n.translate("number.nth.ordinalized", number: number)
end
parameterize(string, separator: "-", preserve_case: false)

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--"
# File activesupport/lib/active_support/inflector/transliterate.rb, line 92
def parameterize(string, separator: "-", preserve_case: false)
  # Replace accented chars with their ASCII equivalents.
  parameterized_string = transliterate(string)

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

  unless separator.nil? || separator.empty?
    if separator == "-".freeze
      re_duplicate_separator        = /-{2,}/
      re_leading_trailing_separator = /^-|-$/i
    else
      re_sep = Regexp.escape(separator)
      re_duplicate_separator        = /#{re_sep}{2,}/
      re_leading_trailing_separator = /^#{re_sep}|#{re_sep}$/i
    end
    # 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, "".freeze)
  end

  parameterized_string.downcase! unless preserve_case
  parameterized_string
end
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 32
def pluralize(word, locale = :en)
  apply_inflections(word, inflections(locale).plurals, locale)
end
safe_constantize(camel_cased_word)

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
end

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 325
def safe_constantize(camel_cased_word)
  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)
end
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 49
def singularize(word, locale = :en)
  apply_inflections(word, inflections(locale).singulars, locale)
end
tableize(class_name)

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 187
def tableize(class_name)
  pluralize(underscore(class_name))
end
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 175
def titleize(word, keep_id_suffix: false)
  humanize(underscore(word), keep_id_suffix: keep_id_suffix).gsub(/\b(?<!\w['’`])[a-z]/) do |match|
    match.capitalize
  end
end
transliterate(string, replacement = "?".freeze)

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

transliterate('Ærøskøbing')
# => "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
i18n:
  transliterate:
    rule:
      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:

I18n.locale = :en
transliterate('Jürgen')
# => "Jurgen"

I18n.locale = :de
transliterate('Jürgen')
# => "Juergen"
# File activesupport/lib/active_support/inflector/transliterate.rb, line 61
def transliterate(string, replacement = "?".freeze)
  raise ArgumentError, "Can only transliterate strings. Received #{string.class.name}" unless string.is_a?(String)

  I18n.transliterate(
    ActiveSupport::Multibyte::Unicode.normalize(
      ActiveSupport::Multibyte::Unicode.tidy_bytes(string), :c),
    replacement: replacement)
end
underscore(camel_cased_word)

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 92
def underscore(camel_cased_word)
  return camel_cased_word unless /[A-Z-]|::/.match?(camel_cased_word)
  word = camel_cased_word.to_s.gsub("::".freeze, "/".freeze)
  word.gsub!(inflections.acronyms_underscore_regex) { "#{$1 && '_'.freeze }#{$2.downcase}" }
  word.gsub!(/([A-Z\d]+)([A-Z][a-z])/, '\1_\2'.freeze)
  word.gsub!(/([a-z\d])([A-Z])/, '\1_\2'.freeze)
  word.tr!("-".freeze, "_".freeze)
  word.downcase!
  word
end
upcase_first(string)

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 156
def upcase_first(string)
  string.length > 0 ? string[0].upcase.concat(string[1..-1]) : ""
end