camelize and underscore are sort of inverse of each other, but not in…

… a mathematical sense [#5174 state:resolved]

activesupport/lib/active_support/inflector/methods.rb

@@ -20,6 +20,11 @@ module Inflector
     #   "active_record".camelize(:lower)        # => "activeRecord"
     #   "active_record/errors".camelize         # => "ActiveRecord::Errors"
     #   "active_record/errors".camelize(:lower) # => "activeRecord::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:
+    #
+    #   "SSLError".underscore.camelize # => "SslError"
     def camelize(lower_case_and_underscored_word, first_letter_in_uppercase = true)
       if first_letter_in_uppercase
         lower_case_and_underscored_word.to_s.gsub(/\/(.?)/) { "::#{$1.upcase}" }.gsub(/(?:^|_)(.)/) { $1.upcase }
@@ -28,13 +33,18 @@ def camelize(lower_case_and_underscored_word, first_letter_in_uppercase = true)
       end
     end
 
-    # The reverse of +camelize+. Makes an underscored, lowercase form from the expression in the string.
+    # Makes an underscored, lowercase form from the expression in the string.
     #
     # Changes '::' to '/' to convert namespaces to paths.
     #
     # Examples:
     #   "ActiveRecord".underscore         # => "active_record"
     #   "ActiveRecord::Errors".underscore # => active_record/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:
+    #
+    #   "SSLError".underscore.camelize # => "SslError"
     def underscore(camel_cased_word)
       word = camel_cased_word.to_s.dup
       word.gsub!(/::/, '/')

railties/guides/source/active_support_core_extensions.textile

@@ -1491,13 +1491,15 @@ end
 
 That may be handy to compute method names in a language that follows that convention, for example JavaScript.
 
+INFO: As a rule of thumb you can think of +camelize+ as the inverse of +underscore+, though there are cases where that does not hold: <tt>"SSLError".underscore.camelize</tt> gives back <tt>"SslError"</tt>.
+
 +camelize+ is aliased to +camelcase+.
 
 NOTE: Defined in +active_support/core_ext/string/inflections.rb+.
 
 h5. +underscore+
 
-The method +underscore+ is the inverse of +camelize+, explained above:
+The method +underscore+ goes the other way around, from camel case to paths:
 
 <ruby>
 "Product".underscore   # => "product"
@@ -1530,6 +1532,8 @@ def load_missing_constant(from_mod, const_name)
 end
 </ruby>
 
+INFO: As a rule of thumb you can think of +underscore+ as the inverse of +camelize+, though there are cases where that does not hold. For example, <tt>"SSLError".underscore.camelize</tt> gives back <tt>"SslError"</tt>.
+
 NOTE: Defined in +active_support/core_ext/string/inflections.rb+.
 
 h5. +titleize+