path: root/railties/guides/source/active_support_overview.textile

h2. Active Support Overview

Active Support is the Rails component responsible for providing Ruby language extensions, utilities, and other transversal stuff. It offers a richer bottom-line at the language level, targeted both at the development of Rails applications, and at the development of Rails itself.

By referring to this guide you will learn:

* The extensions to the Ruby core modules and classes provided by Rails.
* The rest of fundamental libraries available in Rails.

endprologue.

h3. Extensions to All Objects

h4. +blank?+ and +present?+

The following values are considered to be blank in a Rails application:

* +nil+ and +false+,

* strings composed only of whitespace, i.e. matching +/\A\s*\z/+,

* empty arrays and hashes, and

* any other object that responds to +empty?+ and it is empty.

WARNING: Note that numbers are not mentioned, in particular 0 and 0.0 are *not* blank.

For example, this method from +ActionDispatch::Response+ uses +blank?+ to easily be robust to +nil+ and whitespace strings in one shot:

<ruby>
def charset
  charset = String(headers["Content-Type"] || headers["type"]).split(";")[1]
  charset.blank? ? nil : charset.strip.split("=")[1]
end
</ruby>

That's a typical use case for +blank?+.

Here, the method Rails runs to instantiate observers upon initialization has nothing to do if there are none:

<ruby>
def instantiate_observers
  return if @observers.blank?
  # ...
end
</ruby>

The method +present?+ is equivalent to +!blank?+:

<ruby>
assert @response.body.present? # same as !@response.body.blank?
</ruby>

h4. +duplicable?+

A few fundamental objects in Ruby are singletons. For example, in the whole live of a program the integer 1 refers always to the same instance:

<ruby>
1.object_id                 # => 3
Math.cos(0).to_i.object_id  # => 3
</ruby>

Hence, there's no way these objects can be duplicated through +dup+ or +clone+:

<ruby>
true.dup  # => TypeError: can't dup TrueClass
</ruby>

Some numbers which are not singletons are not duplicable either:

<ruby>
0.0.clone        # => allocator undefined for Float
(2**1024).clone  # => allocator undefined for Bignum
</ruby>

Active Support provides +duplicable?+ to programmatically query an object about this property:

<ruby>
"".duplicable?     # => true
false.duplicable?  # => false
</ruby>

By definition all objects are +duplicable?+ except +nil+, +false+, +true+, symbols, numbers, and class objects.

WARNING. Using +duplicable?+ is discouraged because it depends on a hard-coded list. Classes have means to disallow duplication like removing +dup+ and +clone+ or raising exceptions from them, only +rescue+ can tell.

h4. +returning+

The method +returning+ yields its argument to a block and returns it. You tipically use it with a mutable object that gets modified in the block:

<ruby>
def html_options_for_form(url_for_options, options, *parameters_for_url)
  returning options.stringify_keys do |html_options|
    html_options["enctype"] = "multipart/form-data" if html_options.delete("multipart")
    html_options["action"]  = url_for(url_for_options, *parameters_for_url)
  end
end
</ruby>

See also "+Object#tap+":#tap.

h4. +tap+

+Object#tap+ exists in Ruby 1.8.7 and 1.9, and it is defined by Active Support for previous versions. This method yields its receiver to a block and returns it.

For example, the following class method from +ActionDispatch::TestResponse+ creates, initializes, and returns a new test response using +tap+:

<ruby>
def self.from_response(response)
  new.tap do |resp|
    resp.status  = response.status
    resp.headers = response.headers
    resp.body    = response.body
  end
end
</ruby>

See also "+Object#returning+":#returning.

h4. +try+

Sometimes you want to call a method provided the receiver object is not +nil+, which is something you usually check first.

For instance, note how this method of +ActiveRecord::ConnectionAdapters::AbstractAdapter+ checks if there's a +@logger+:

<ruby>
def log_info(sql, name, ms)
  if @logger && @logger.debug?
    name = '%s (%.1fms)' % [name || 'SQL', ms]
    @logger.debug(format_log_entry(name, sql.squeeze(' ')))
  end
end
</ruby>

You can shorten that using +Object#try+. This method is a synonim for +Object#send+ except that it returns +nil+ if sent to +nil+. The previous example could then be rewritten as:

<ruby>
def log_info(sql, name, ms)
  if @logger.try(:debug?)
    name = '%s (%.1fms)' % [name || 'SQL', ms]
    @logger.debug(format_log_entry(name, sql.squeeze(' ')))
  end
end
</ruby>

h4. +metaclass+

The method +metaclass+ returns the singleton class on any object:

<ruby>
String.metaclass     # => #<Class:String>
String.new.metaclass # => #<Class:#<String:0x17a1d1c>>
</ruby>

h4. +class_eval(*args, &block)+

You can evaluate code in the context of any object's singleton class using +class_eval+:

<ruby>
class Proc
  def bind(object)
    block, time = self, Time.now
    object.class_eval do
      method_name = "__bind_#{time.to_i}_#{time.usec}"
      define_method(method_name, &block)
      method = instance_method(method_name)
      remove_method(method_name)
      method
    end.bind(object)
  end
end
</ruby>

h4. +acts_like?(duck)+

The method +acts_like+ provides a way to check whether some class acts like some other class based on a simple convention: a class that provides the same interface as +String+ defines

<ruby>
def acts_like_string?
end
</ruby>

which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:

<ruby>
some_klass.acts_like?(:string)
</ruby>

Rails has classes that act like +Date+ or +Time+ and follow this contract.

h4. +to_param+

All objects in Rails respond to the method +to_param+, which is meant to return something that represents them as values in a query string, or as a URL fragments.

By default +to_param+ just calls +to_s+:

<ruby>
7.to_param # => "7"
</ruby>

The return value of +to_param+ should *not* be escaped:

<ruby>
"Tom & Jerry".to_param # => "Tom & Jerry"
</ruby>

Several classes in Rails overwrite this method.

For example +nil+, +true+, and +false+ return themselves. +Array#to_param+ calls +to_param+ on the elements and joins the result with "/":

<ruby>
[0, true, String].to_param # => "0/true/String"
</ruby>

Notably, the Rails routing system calls +to_param+ on models to get a value for the +:id+ placeholder. +ActiveRecord::Base#to_param+ returns the +id+ of a model, but you can redefine that method in your models. For example, given

<ruby>
class User
  def to_param
    "#{id}-#{name.parameterize}"
  end
end
</ruby>

we get:

<ruby>
user_path(@user) # => "/users/357-john-smith"
</ruby>

WARNING. Controllers need to be aware of any redifinition of +to_param+ because when a request like that comes in "357-john-smith" is the value of +params[:id]+.

h4. +to_query+

Except for hashes, given an unescaped +key+ this method constructs the part of a query string that would map such key to what +to_param+ returns. For example, given

<ruby>
class User
  def to_param
    "#{id}-#{name.parameterize}"
  end
end
</ruby>

we get:

<ruby>
current_user.to_query('user') # => user=357-john-smith
</ruby>

This method escapes whatever is needed, both for the key and the value:

<ruby>
account.to_query('company[name]')
# => "company%5Bname%5D=Johnson+%26+Johnson"
</ruby>

so its output is ready to be used in a query string.

Arrays return the result of applying +to_query+ to each element with <tt>_key_[]</tt> as key, and join the result with "/":

<ruby>
[3.4, -45.6].to_query('sample')
# => "sample%5B%5D=3.4&sample%5B%5D=-45.6"
</ruby>

Hashes also respond to +to_query+ but with a different signature. If no argument is passed a call generates a sorted series of key/value assigments calling +to_query(key)+ on its values. Then it joins the result with "&":

<ruby>
{:c => 3, :b => 2, :a => 1}.to_query # => "a=1&b=2&c=3"
</ruby>

The method +Hash#to_query+ accepts an optional namespace for the keys:

<ruby>
{:id => 89, :name => "John Smith"}.to_query('user')
# => "user%5Bid%5D=89&user%5Bname%5D=John+Smith"
</ruby>

h4. +with_options+

The method +with_options+ provides a way to factor out common options in a series of method calls.

Given a default options hash, +with_options+ yields a proxy object to a block. Within the block, methods called on the proxy are forwarded to the receiver with their options merged. For example, you get rid of the duplication in:

<ruby>
class Account < ActiveRecord::Base
  has_many :customers, :dependent => :destroy
  has_many :products,  :dependent => :destroy
  has_many :invoices,  :dependent => :destroy
  has_many :expenses,  :dependent => :destroy
end
</ruby>

this way:

<ruby>
class Account < ActiveRecord::Base
  with_options :dependent => :destroy do |assoc|
    assoc.has_many :customers
    assoc.has_many :products
    assoc.has_many :invoices
    assoc.has_many :expenses
  end
end
</ruby>

That idiom may convey _grouping_ to the reader as well. For example, say you want to send a newsletter whose language depends on the user. Somewhere in the mailer you could group locale-dependent bits like this:

<ruby>
I18n.with_options :locale => user.locale, :scope => "newsletter" do |i18n|
  subject i18n.t :subject
  body    i18n.t :body, :user_name => user.name 
end
</ruby>

TIP: Since +with_options+ forwards calls to its receiver they can be nested. Each nesting level will merge inherited defaults in addition to their own.

h4. Instance Variables

Active Support provides several methods to ease access to instance variables.

h5. +instance_variable_defined?+

The method +instance_variable_defined?+ exists in Ruby 1.8.6 and later, and it is defined for previous versions anyway:

<ruby>
class C
  def initialize
    @a = 1
  end

  def m
    @b = 2
  end
end

c = C.new

c.instance_variable_defined?("@a") # => true
c.instance_variable_defined?(:@a)  # => true
c.instance_variable_defined?("a")  # => NameError: `a' is not allowed as an instance variable name

c.instance_variable_defined?("@b") # => false
c.m
c.instance_variable_defined?("@b") # => true
</ruby>

h5. +instance_variable_names+

Ruby 1.8 and 1.9 have a method called +instance_variables+ that returns the names of the defined instance variables. But they behave differently, in 1.8 it returns strings whereas in 1.9 it returns symbols. Active Support defines +instance_variable_names+ as a portable way to obtain them as strings:

<ruby>
class C
  def initialize(x, y)
    @x, @y = x, y
  end
end

C.new(0, 1).instance_variable_names # => ["@y", "@x"]
</ruby>

WARNING: The order in which the names are returned is unespecified, and it indeed depends on the version of the interpreter.

h5. +instance_values+

The method +instance_values+ returns a hash that maps instance variable names without "@" to their
corresponding values. Keys are strings both in Ruby 1.8 and 1.9:

<ruby>
class C
  def initialize(x, y)
    @x, @y = x, y
  end
end

C.new(0, 1).instance_values # => {"x" => 0, "y" => 1}
</ruby>

h5. +copy_instance_variables_from(object, exclude = [])+

Copies the instance variables of +object+ into +self+.

Instance variable names in the +exclude+ array are ignored. If +object+
responds to +protected_instance_variables+ the ones returned are
also ignored. For example, Rails controllers implement that method.

In both arrays strings and symbols are understood, and they have to include
the at sign.

<ruby>
class C
  def initialize(x, y, z)
    @x, @y, @z = x, y, z
  end

  def protected_instance_variables
    %w(@z)
  end
end

a = C.new(0, 1, 2)
b = C.new(3, 4, 5)

a.copy_instance_variables_from(b, [:@y])
# a is now: @x = 3, @y = 1, @z = 2
</ruby>

In the example +object+ and +self+ are of the same type, but they don't need to.

h4. Silencing Warnings, Streams, and Exceptions

The methods +silence_warnings+ and +enable_warnings+ change the value of +$VERBOSE+ accordingly for the duration of their block, and reset it afterwards:

<ruby>
silence_warnings { Object.const_set "RAILS_DEFAULT_LOGGER", logger }
</ruby>

You can silence any stream while a block runs with +silence_stream+:

<ruby>
silence_stream(STDOUT) do
  # STDOUT is silent here
end
</ruby>

Silencing exceptions is also possible with +suppress+. This method receives an arbitrary number of exception classes. If an exception is raised during the execution of the block and is +kind_of?+ any of the arguments, +suppress+ captures it and returns silently. Otherwise the exception is reraised:

<ruby>
# If the user is locked the increment is lost, no big deal.
suppress(ActiveRecord::StaleObjectError) do
  current_user.increment! :visits
end
</ruby>

h3. Extensions to +Module+

...

h3. Extensions to +Class+

h4. Class Attribute Accessors

The macros +cattr_reader+, +cattr_writer+, and +cattr_accessor+ are analogous to their +attr_*+ counterparts but for classes. They initialize a class variable to +nil+ unless it already exists, and generate the corresponding class methods to access it:

<ruby>
class MysqlAdapter < AbstractAdapter
  # Generates class methods to access @@emulate_booleans.
  cattr_accessor :emulate_booleans
  self.emulate_booleans = true
end
</ruby>

Instance methods are created as well for convenience. For example given

<ruby>
module ActionController
  class Base
    cattr_accessor :logger
  end
end
</ruby>

we can access +logger+ in actions. The generation of the writer instance method can be prevented setting +:instance_writer+ to +false+ (not any false value, but exactly +false+):

<ruby>
module ActiveRecord
  class Base
    # No pluralize_table_names= instance writer is generated.
    cattr_accessor :pluralize_table_names, :instance_writer => false
  end
end
</ruby>

h4. Class Inheritable Attributes

Class variables are shared down the inheritance tree. Class instance variables are not shared, but they are not inherited either. The macros +class_inheritable_reader+, +class_inheritable_writer+, and +class_inheritable_accessor+ provide accesors for class-level data which is inherited but not shared with children:

<ruby>
module ActionController
  class Base
    # FIXME: REVISE/SIMPLIFY THIS COMMENT.
    # The value of allow_forgery_protection is inherited,
    # but its value in a particular class does not affect
    # the value in the rest of the controllers hierarchy.
    class_inheritable_accessor :allow_forgery_protection
  end
end
</ruby>

They accomplish this with class instance variables and cloning on subclassing, there are no class variables involved. Cloning is performed with +dup+ as long as the value is duplicable.

There are some variants specialised in arrays and hashes:

<ruby>
class_inheritable_array
class_inheritable_hash
</ruby>

Those writers take any inherited array or hash into account and extend them rather than overwrite them.

As with vanilla class attribute accessors these macros create convenience instance methods for reading and writing. The generation of the writer instance method can be prevented setting +:instance_writer+ to +false+ (not any false value, but exactly +false+):

<ruby>
module ActiveRecord
  class Base
    class_inheritable_accessor :default_scoping, :instance_writer => false
  end
end
</ruby>

Since values are copied when a subclass is defined, if the base class changes the attribute after that, the subclass does not see the new value. That's the point. 

There's a related macro called +superclass_delegating_accessor+, however, that does not copy the value when the base class is subclassed. Instead, it delegates reading to the superclass as long as the attribute is not set via its own writer. For example, +ActionMailer::Base+ defines +delivery_method+ this way:

<ruby>
module ActionMailer
  class Base
    superclass_delegating_accessor :delivery_method
    self.delivery_method = :smtp
  end
end
</ruby>

If for whatever reason an application loads the definition of a mailer class and after that sets +ActionMailer::Base.delivery_method+, the mailer class will still see the new value. In addition, the mailer class is able to change the +delivery_method+ without affecting the value in the parent using its own inherited class attribute writer.

h4. Subclasses

The +subclasses+ method returns the names of all subclasses of a given class as an array of strings. That comprises not only direct subclasses, but all descendants down the hierarchy:

<ruby>
class C; end
C.subclasses # => []

Integer.subclasses # => ["Bignum", "Fixnum"]

module M
  class A; end
  class B1 < A; end
  class B2 < A; end
end

module N
  class C < M::B1; end
end

M::A.subclasses # => ["N::C", "M::B2", "M::B1"]
</ruby>

The order in which these class names are returned is unspecified.

See also +Object#subclasses_of+ in "Extensions to All Objects FIX THIS LINK":FIXME.

h4. Class Removal

Roughly speaking, the +remove_class+ method removes the class objects passed as arguments:

<ruby>
Class.remove_class(Hash, Dir) # => [Hash, Dir]
Hash # => NameError: uninitialized constant Hash
Dir  # => NameError: uninitialized constant Dir
</ruby>

More specifically, +remove_class+ attempts to remove constants with the same name as the passed class objects from their parent modules. So technically this method does not guarantee the class objects themselves are not still valid and alive somewhere after the method call:

<ruby>
module M
  class A; end
  class B < A; end
end

A2 = M::A

M::A.object_id            # => 13053950
Class.remove_class(M::A)

M::B.superclass.object_id # => 13053950 (same object as before)
A2.name                   # => "M::A" (name is hard-coded in object)
</ruby>

WARNING: Removing fundamental classes like +String+ can result in really funky behaviour.

The method +remove_subclasses+ provides a shortcut for removing all descendants of a given class, where "removing" has the meaning explained above:

<ruby>
class A; end
class B1 < A; end
class B2 < A; end
class C < A; end

A.subclasses        # => ["C", "B2", "B1"]
A.remove_subclasses
A.subclasses        # => []
C                   # => NameError: uninitialized constant C
</ruby>

See also +Object#remove_subclasses_of+ in "Extensions to All Objects FIX THIS LINK":FIXME.

h3. Extensions to +Symbol+

h4. +to_proc+

The method +to_proc+ turns a symbol into a Proc object so that for example

<ruby>
emails = users.map {|u| u.email}
</ruby>

can be written as

<ruby>
emails = users.map(&:email)
</ruby>

TIP: If the method that receives the Proc yields more than one value to it the rest are considered to be arguments of the method call.

Symbols from Ruby 1.8.7 on respond to +to_proc+, and Active Support defines it for previous versions.

h3. Extensions to +String+

...

h3. Extensions to +Numeric+

...

h3. Extensions to +Integer+

h4. +multiple_of?+

The method +multiple_of?+ tests whether an integer is multiple of the argument:

<ruby>
2.multiple_of?(1) # => true
1.multiple_of?(2) # => false
</ruby>

WARNING: Due the way it is implemented the argument must be nonzero, otherwise +ZeroDivisionError+ is raised.

h4. +even?+ and +odd?+

Integers in Ruby 1.8.7 and above respond to +even?+ and +odd?+, Active Support defines them for older versions:

<ruby>
-1.even?  # => false
-1.odd?   # => true
 0.even?  # => true
 0.odd?   # => false
 2.even?  # => true
 2.odd?   # => false
</ruby>

h4. +ordinalize+

The method +ordinalize+ returns the ordinal string corresponding to the receiver integer:

<ruby>
1.ordinalize    # => "1st"
2.ordinalize    # => "2nd"
53.ordinalize   # => "53rd"
2009.ordinalize # => "2009th"
</ruby>

h3. Extensions to +Float+

...

h3. Extensions to +BigDecimal+

...

h3. Extensions to +Enumerable+

...

h3. Extensions to +Array+

h4. Accessing

Active Support augments the API of arrays to ease certain ways of accessing them. For example, +to+ returns the subarray of elements up to the one at the passed index:

<ruby>
%w(a b c d).to(2) # => %w(a b c)
[].to(7)          # => []
</ruby>

Similarly, +from+ returns the tail from the element at the passed index on:

<ruby>
%w(a b c d).from(2)  # => %w(c d)
%w(a b c d).from(10) # => nil
[].from(0)           # => nil
</ruby>

The methods +second+, +third+, +fourth+, and +fifth+ return the corresponding element (+first+ is builtin). Thanks to social wisdom and positive constructiveness all around, +forty_two+ is also available.

You can pick a random element with +rand+:

<ruby>
shape_type = [Circle, Square, Triangle].rand
</ruby>

h4. Grouping

h5. +in_groups_of(number, fill_with = nil)+

The method +in_groups_of+ splits an array into consecutive groups of a certain size. It returns an array with the groups:

<ruby>
[1, 2, 3].in_groups_of(2) # => [[1, 2], [3, nil]]
</ruby>

or yields them in turn if a block is passed:

<ruby>
<% sample.in_groups_of(3) do |a, b, c| %>
  <tr>
    <td><%=h a %></td>
    <td><%=h b %></td>
    <td><%=h c %></td>
  </tr>
<% end %>
</ruby>

The first example shows +in_groups_of+ fills the last group with as many +nil+ elements as needed to have the requested size. You can change this padding value using the second optional argument:

<ruby>
[1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]
</ruby>

And you can tell the method not to fill the last group passing +false+:

<ruby>
[1, 2, 3].in_groups_of(2, false) # => [[1, 2], [3]]
</ruby>

As a consequence +false+ can't be a used as a padding value.

h5. +in_groups(number, fill_with = nil)+

The method +in_groups+ splits an array into a certain number of groups. The method returns and array with the groups:

<ruby>
%w(1 2 3 4 5 6 7).in_groups(3)
# => [["1", "2", "3"], ["4", "5", nil], ["6", "7", nil]]
</ruby>

or yields them in turn if a block is passed:

<ruby>
%w(1 2 3 4 5 6 7).in_groups(3) {|group| p group}
["1", "2", "3"]
["4", "5", nil]
["6", "7", nil]
</ruby>

The examples above show that +in_groups+ fills some groups with a trailing +nil+ element as needed. A group can get at most one of these extra elements, the rightmost one if any. And the groups that have them are always the last ones.

You can change this padding value using the second optional argument:

<ruby>
%w(1 2 3 4 5 6 7).in_groups(3, "0")
# => [["1", "2", "3"], ["4", "5", "0"], ["6", "7", "0"]]
</ruby>

And you can tell the method not to fill the smaller groups passing +false+:

<ruby>
%w(1 2 3 4 5 6 7).in_groups(3, false)
# => [["1", "2", "3"], ["4", "5"], ["6", "7"]]
</ruby>

As a consequence +false+ can't be a used as a padding value.

h5. +split(value = nil)+

The method +split+ divides an array by a separator and returns the resulting chunks.

If a block is passed the separators are those elements of the array for which the block returns true:

<ruby>
(-5..5).to_a.split { |i| i.multiple_of?(4) }
# => [[-5], [-3, -2, -1], [1, 2, 3], [5]]
</ruby>

Otherwise, the value received as argument, which defaults to +nil+, is the separator:

<ruby>
[0, 1, -5, 1, 1, "foo", "bar"].split(1)
# => [[0], [-5], [], ["foo", "bar"]]
</ruby>

NOTE: Observe in the previous example that consecutive separators result in empty arrays.

h3. Extensions to +Hash+

...

h3. Extensions to +Range+

...

h3. Extensions to +Proc+

...

h3. Extensions to +Date+

...

h3. Extensions to +DateTime+

...

h3. Extensions to +Time+

...

h3. Extensions to +Process+

...

h3. Extensions to +Pathname+

...

h3. Extensions to +File+

h4. +atomic_write+

With the class method +File.atomic_write+ you can write to a file in a way that will prevent any reader from seeing half-written content.

The name of the file is passed as an argument, and the method yields a file handle opened for writing. Once the block is done +atomic_write+ closes the file handle and completes its job.

For example, Action Pack uses this method to write asset cache files like +all.css+:

<ruby>
File.atomic_write(joined_asset_path) do |cache|
  cache.write(join_asset_file_contents(asset_paths))
end
</ruby>

To accomplish this +atomic_write+ creates a temporary file. That's the file the code in the block actually writes to. On completion, the temporary file is renamed. If the target file exists +atomic_write+ overwrites it and keeps owners and permissions.

WARNING. Note you can't append with +atomic_write+.

The auxiliary file is written in a standard directory for temporary files, but you can pass a directory of your choice as second argument.

h3. Extensions to +Exception+

...

h3. Extensions to +NameError+

Active Support adds +missing_name?+ to +NameError+, which tests whether the exception was raised because of the name passed as argument.

The name may be given as a symbol or string. A symbol is tested against the bare constant name, a string is against the fully-qualified constant name.

TIP: A symbol can represent a fully-qualified constant name as in +:"ActiveRecord::Base"+, so the behaviour for symbols is defined for convenience, not because it has to be that way technically.

For example, when an action of +PostsController+ is called Rails tries optimistically to use +PostsHelper+. It is OK that the helper module does not exist, so if an exception for that constant name is raised it should be silenced. But it could be the case that +posts_helper.rb+ raises a +NameError+ due to an actual unknown constant. That should be reraised. The method +missing_name?+ provides a way to distinguish both cases:

<ruby>
def default_helper_module!
  module_name = name.sub(/Controller$/, '')
  module_path = module_name.underscore
  helper module_path
rescue MissingSourceFile => e
  raise e unless e.is_missing? "#{module_path}_helper"
rescue NameError => e
  raise e unless e.missing_name? "#{module_name}Helper"
end
</ruby>
 
h3. Extensions to +LoadError+

Rails hijacks +LoadError.new+ to return a +MissingSourceFile+ exception:

<shell>
$ ruby -e 'require "nonexistent"'
...: no such file to load -- nonexistent (LoadError)
...
$ script/runner 'require "nonexistent"'
...: no such file to load -- nonexistent (MissingSourceFile)
...
</shell>

The class +MissingSourceFile+ is a subclass of +LoadError+, so any code that rescues +LoadError+ as usual still works as expected. Point is these exception objects respond to +is_missing?+, which given a path name tests whether the exception was raised due to that particular file (except perhaps for the ".rb" extension).

For example, when an action of +PostsController+ is called Rails tries to load +posts_helper.rb+, but that file may not exist. That's fine, the helper module is not mandatory so Rails silences a load error. But it could be the case that the helper module does exist, but it in turn requires another library that is missing. In that case Rails must reraise the exception. The method +is_missing?+ provides a way to distinguish both cases:

<ruby>
def default_helper_module!
  module_name = name.sub(/Controller$/, '')
  module_path = module_name.underscore
  helper module_path
rescue MissingSourceFile => e
  raise e unless e.is_missing? "#{module_path}_helper"
rescue NameError => e
  raise e unless e.missing_name? "#{module_name}Helper"
end
</ruby>

h3. Extensions to +CGI+

...

h3. Extensions to +Benchmark+

...

h3. Changelog

"Lighthouse ticket":https://rails.lighthouseapp.com/projects/16213/tickets/67

* April 18, 2009: Initial version by "Xavier Noria":credits.html#fxn