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 classes and modules provided by Rails.
endprologue.
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:
def charset
charset = String(headers[“Content-Type”] || headers[“type”]).split(“;”)1
charset.blank? ? nil : charset.strip.split(“=”)1
end
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:
def instantiate_observers
return if @observers.blank?
- …
end
The method present? is equivalent to !blank?:
assert response.body.present? # same as !response.body.blank?
NOTE: Defined in active_support/core_ext/object/blank.rb.
The presence method returns its receiver if present?, and nil otherwise. It is useful for idioms like this:
host = config[:host].presence || ‘localhost’
NOTE: Defined in active_support/core_ext/object/blank.rb.
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:
1.object_id # => 3
Math.cos(0).to_i.object_id # => 3
Hence, there’s no way these objects can be duplicated through dup or clone:
true.dup # => TypeError: can’t dup TrueClass
Some numbers which are not singletons are not duplicable either:
0.0.clone # => allocator undefined for Float
(2**1024).clone # => allocator undefined for Bignum
Active Support provides duplicable? to programmatically query an object about this property:
"".duplicable? # => true
false.duplicable? # => false
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.
NOTE: Defined in active_support/core_ext/object/duplicable.rb.
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:
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
NOTE: Defined in active_support/core_ext/object/returning.rb.
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:
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
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:
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
NOTE: Defined in active_support/core_ext/object/try.rb.
The method metaclass returns the singleton class on any object:
String.metaclass # => #
String.new.metaclass # => #<Class:#>
NOTE: Defined in active_support/core_ext/object/metaclass.rb.
You can evaluate code in the context of any object’s singleton class using class_eval:
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
NOTE: Defined in active_support/core_ext/object/metaclass.rb.
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
def acts_like_string?
end
which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:
some_klass.acts_like?(:string)
Rails has classes that act like Date or Time and follow this contract.
NOTE: Defined in active_support/core_ext/object/acts_like.rb.
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:
7.to_param # => “7”
The return value of to_param should not be escaped:
“Tom & Jerry”.to_param # => “Tom & Jerry”
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 “/”:
[0, true, String].to_param # => “0/true/String”
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
class User
def to_param
“#{id}-#{name.parameterize}”
end
end
we get:
user_path(@user) # => “/users/357-john-smith”
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].
NOTE: Defined in active_support/core_ext/object/to_param.rb.
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
class User
def to_param
“#{id}-#{name.parameterize}”
end
end
we get:
current_user.to_query(‘user’) # => user=357-john-smith
This method escapes whatever is needed, both for the key and the value:
account.to_query(‘company[name]’)
- => “company%5Bname%5D=Johnson+%26+Johnson”
so its output is ready to be used in a query string.
Arrays return the result of applying to_query to each element with key[] as key, and join the result with “&”:
[3.4, -45.6].to_query(‘sample’)
- => “sample%5B%5D=3.4&sample%5B%5D=-45.6”
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 “&”:
{:c => 3, :b => 2, :a => 1}.to_query # => “a=1&b=2&c=3”
The method Hash#to_query accepts an optional namespace for the keys:
{:id => 89, :name => “John Smith”}.to_query(‘user’)
- => “user%5Bid%5D=89&user%5Bname%5D=John+Smith”
NOTE: Defined in active_support/core_ext/object/to_query.rb.
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:
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
this way:
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
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:
I18n.with_options :locale => user.locale, :scope => “newsletter” do |i18n|
subject i18n.t :subject
body i18n.t :body, :user_name => user.name
end
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.
NOTE: Defined in active_support/core_ext/object/with_options.rb.
The method remove_subclasses_of receives an arbitrary number of class objects and removes their subclasses. It is a wrapper of Class#remove_class explained with more details in Class Removal FIX THIS LINK.
The method subclasses_of receives an arbitrary number of class objects and returns all their anonymous or reachable descendants as a single array:
class C; end
subclasses_of© # => []
subclasses_of(Integer) # => [Bignum, Fixnum]
module M
class A; end
class B1 < A; end
class B2 < A; end
end
module N
class C < M::B1; end
end
subclasses_of(M::A) # => [N::C, M::B2, M::B1]
The order in which these classes are returned is unspecified. The returned collection may have duplicates:
subclasses_of(Numeric, Integer)
- => [Bignum, Float, Fixnum, Integer, Date::Infinity, Rational, BigDecimal, Bignum, Fixnum]
See also Class#subclasses in Extensions to Class FIXME THIS LINK.
NOTE: Defined in active_support/core_ext/object/extending.rb.
Active Support provides several methods to ease access to instance variables.
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:
class C
def initialize(x, y)
@x, @y = x, y
end
end
C.new(0, 1).instance_variable_names # => [“@y”, “@x”]
WARNING: The order in which the names are returned is unespecified, and it indeed depends on the version of the interpreter.
NOTE: Defined in active_support/core_ext/object/instance_variables.rb.
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:
class C
def initialize(x, y)
@x, @y = x, y
end
end
C.new(0, 1).instance_values # => {"x" => 0, “y” => 1}
NOTE: Defined in active_support/core_ext/object/instance_variables.rb.
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.
class C
def initialize(x, y, z)
@x, @y, @z = x, y, 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
In the example object and self are of the same type, but they don’t need to.
NOTE: Defined in active_support/core_ext/object/instance_variables.rb.
The methods silence_warnings and enable_warnings change the value of $VERBOSE accordingly for the duration of their block, and reset it afterwards:
silence_warnings { Object.const_set “RAILS_DEFAULT_LOGGER”, logger }
You can silence any stream while a block runs with silence_stream:
silence_stream(STDOUT) do
- STDOUT is silent here
end
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:
- If the user is locked the increment is lost, no big deal.
suppress(ActiveRecord::StaleObjectError) do
current_user.increment! :visits
end
NOTE: Defined in active_support/core_ext/kernel/reporting.rb.
Using plain Ruby you can wrap methods with other methods, that’s called alias chaining.
For example, let’s say you’d like params to be strings in functional tests, as they are in real requests, but still want the convenience of assigning integers and other kind of values. To accomplish that you could wrap ActionController::TestCase#process this way in test/test_helper.rb:
ActionController::TestCase.class_eval do
- save a reference to the original process method
alias_method :original_process, :process
- now redefine process and delegate to original_process
def process(action, params=nil, session=nil, flash=nil, http_method=‘GET’)
params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
original_process(action, params, session, flash, http_method)
end
end
That’s the method get, post, etc., delegate the work to.
That technique has a risk, it could be the case that :original_process was taken. To try to avoid collisions people choose some label that characterizes what the chaining is about:
ActionController::TestCase.class_eval do
def process_with_stringified_params(…)
params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
process_without_stringified_params(action, params, session, flash, http_method)
end
alias_method :process_without_stringified_params, :process
alias_method :process, :process_with_stringified_params
end
The method alias_method_chain provides a shortcut for that pattern:
ActionController::TestCase.class_eval do
def process_with_stringified_params(…)
params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
process_without_stringified_params(action, params, session, flash, http_method)
end
alias_method_chain :process, :stringified_params
end
Rails uses alias_method_chain all over the code base. For example validations are added to ActiveRecord::Base#save by wrapping the method that way in a separate module specialised in validations.
NOTE: Defined in active_support/core_ext/module/aliasing.rb.
Model attributes have a reader, a writer, and a predicate. You can aliase a model attribute having the corresponding three methods defined for you in one shot. As in other aliasing methods, the new name is the first argument, and the old name is the second (my mnemonic is they go in the same order as if you did an assignment):
class User < ActiveRecord::Base
- let me refer to the email column as “login”,
- much meaningful for authentication code
alias_attribute :login, :email
end
NOTE: Defined in active_support/core_ext/module/aliasing.rb.
The class method delegate
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:
class MysqlAdapter < AbstractAdapter
- Generates class methods to access @@emulate_booleans.
cattr_accessor :emulate_booleans
self.emulate_booleans = true
end
Instance methods are created as well for convenience. For example given
module ActionController
class Base
cattr_accessor :logger
end
end
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):
module ActiveRecord
class Base
- No pluralize_table_names= instance writer is generated.
cattr_accessor :pluralize_table_names, :instance_writer => false
end
end
NOTE: Defined in active_support/core_ext/class/attribute_accessors.rb.
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:
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
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:
class_inheritable_array
class_inheritable_hash
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):
module ActiveRecord
class Base
class_inheritable_accessor :default_scoping, :instance_writer => false
end
end
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.
NOTE: Defined in active_support/core_ext/class/inheritable_attributes.rb.
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:
module ActionMailer
class Base
superclass_delegating_accessor :delivery_method
self.delivery_method = :smtp
end
end
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.
NOTE: Defined in active_support/core_ext/class/delegating_attributes.rb.
The descendents method returns all the descendants of its receiver as an array of class objects. This method performs no filtering so non-reachable classes are included, if any.
See aso Object#subclasses_of, explained in Extensions to All Objects FIX THIS LINK.
NOTE: Defined in active_support/core_ext/object/extending.rb.
The subclasses method returns the names of all the anonymous or reachable descendants of its receiver as an array of strings:
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”]
WARNING: ActiveRecord::Base redefines subclasses, it returns class objects, reachable or not, and it is protected.
See aso Object#subclasses_of, explained in Extensions to All Objects FIX THIS LINK.
NOTE: Defined in active_support/core_ext/class/removal.rb.
Roughly speaking, the remove_class method removes the class objects passed as arguments:
Class.remove_class(Hash, Dir) # => [Hash, Dir]
Hash # => NameError: uninitialized constant Hash
Dir # => NameError: uninitialized constant Dir
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:
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)
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:
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
See also Object#remove_subclasses_of in Extensions to All Objects FIX THIS LINK.
NOTE: Defined in active_support/core_ext/class/removal.rb.
By definition a non-anonymous class is reachable if its name constantized is defined, and the corresponding constant evaluates to self:
class C; end
C.reachable? # => true
phantom = Object.send(:remove_const, :C)
- The class object is orphan now but it still has a name.
phantom.name # => “C”
- Class name no longer available as a constant.
phantom.reachable? # => nil
- Let’s define a class named “C” again.
class C; end
- Class name available as a constant, but different class object.
phantom.reachable? # => false
NOTE: Defined in active_support/core_ext/class/removal.rb.
The method String#squish strips leading and trailing whitespace, and substitutes runs of whitespace with a single space each:
" \n foo\n\r \t bar \n".squish # => “foo bar”
There’s also the destructive version String#squish!.
NOTE: Defined in active_support/core_ext/string/filters.rb.
In Ruby 1.9 the % string operator supports key-based interpolation, both formatted and unformatted:
“Total is %.02f” % {:total => 43.1} # => Total is 43.10
“I say %{foo}” % {:foo => "wadus"} # => “I say wadus”
“I say %{woo}” % {:foo => "wadus"} # => KeyError
Active Support adds that functionality to % in previous versions of Ruby.
NOTE: Defined in active_support/core_ext/string/interpolation.rb.
Active Support defines 3rd person aliases of String#start_with? and String#end_with?:
“foo”.starts_with?(“f”) # => true
“foo”.ends_with?(“o”) # => true
NOTE: Defined in active_support/core_ext/string/starts_ends_with.rb.
Returns the character of the string at position position:
“hello”.at(0) # => “h”
“hello”.at(4) # => “o”
“hello”.at(-1) # => “o”
“hello”.at(10) # => ERROR if < 1.9, nil in 1.9
NOTE: Defined in active_support/core_ext/string/access.rb.
Returns the substring of the string starting at position position:
“hello”.from(0) # => “hello”
“hello”.from(2) # => “llo”
“hello”.from(-2) # => “lo”
“hello”.from(10) # => "" if < 1.9, nil in 1.9
NOTE: Defined in active_support/core_ext/string/access.rb.
Returns the substring of the string up to position position:
“hello”.to(0) # => “h”
“hello”.to(2) # => “hel”
“hello”.to(-2) # => “hell”
“hello”.to(10) # => “hello”
NOTE: Defined in active_support/core_ext/string/access.rb.
The call str.first(n) is equivalent to str.to(n-1) if n > 0, and returns an empty string for n == 0.
NOTE: Defined in active_support/core_ext/string/access.rb.
The call str.last(n) is equivalent to str.from(-n) if n > 0, and returns an empty string for n == 0.
NOTE: Defined in active_support/core_ext/string/access.rb.
All numbers respond to these methods:
bytes
kilobytes
megabytes
gigabytes
terabytes
petabytes
exabytes
They return the corresponding amount of bytes, using a conversion factor of 1024:
2.kilobytes # => 2048
3.megabytes # => 3145728
3.5.gigabytes # => 3758096384
-4.exabytes # => -4611686018427387904
Singular forms are aliased so you are able to say:
1.megabyte # => 1048576
NOTE: Defined in active_support/core_ext/numeric/bytes.rb.
The method multiple_of? tests whether an integer is multiple of the argument:
2.multiple_of?(1) # => true
1.multiple_of?(2) # => false
NOTE: Defined in active_support/core_ext/integer/multiple.rb.
The method ordinalize returns the ordinal string corresponding to the receiver integer:
1.ordinalize # => “1st”
2.ordinalize # => “2nd”
53.ordinalize # => “53rd”
2009.ordinalize # => “2009th”
NOTE: Defined in active_support/core_ext/integer/inflections.rb.
…
…
Ruby 1.8.7 and up define group_by, and Active Support does it for previous versions.
This iterator takes a block and builds an ordered hash with its return values as keys. Each key is mapped to the array of elements for which the block returned that value:
entries_by_surname_initial = address_book.group_by do |entry|
entry.surname.at(0).upcase
end
WARNING. Active Support redefines group_by in Ruby 1.8.7 so that it still returns an ordered hash.
NOTE: Defined in active_support/core_ext/enumerable.rb.
The method sum adds the elements of an enumerable:
[1, 2, 3].sum # => 6
(1..100).sum # => 5050
Addition only assumes the elements respond to +:
[[1, 2], [2, 3], [3, 4]].sum # => [1, 2, 2, 3, 3, 4]
%w(foo bar baz).sum # => “foobarbaz”
{:a => 1, :b => 2, :c => 3}.sum # => [:b, 2, :c, 3, :a, 1]
The sum of an empty collection is zero by default, but this is customizable:
[].sum # => 0
[].sum(1) # => 1
If a block is given sum becomes an iterator that yields the elements of the collection and sums the returned values:
(1..5).sum {|n| n * 2 } # => 30
[2, 4, 6, 8, 10].sum # => 30
The sum of an empty receiver can be customized in this form as well:
[].sum(1) {|n| n**3} # => 1
The method ActiveRecord::Observer#observed_subclasses for example is implemented this way:
def observed_subclasses
observed_classes.sum([]) { |klass| klass.send(:subclasses) }
end
NOTE: Defined in active_support/core_ext/enumerable.rb.
The inject method offers iteration with an accumulator:
[2, 3, 4].inject(1) {|acc, i| product*i } # => 24
The block is expected to return the value for the accumulator in the next iteration, and this makes building mutable objects a bit cumbersome:
[1, 2].inject({}) {|h, i| h[i] = i**2; h} # => {1 => 1, 2 => 4}
See that spurious “; h”?
Active Support backports each_with_object from Ruby 1.9, which addresses that use case. It iterates over the collection, passes the accumulator, and returns the accumulator when done. You normally modify the accumulator in place. The example above would be written this way:
[1, 2].each_with_object({}) {|i, h| h[i] = i**2} # => {1 => 1, 2 => 4}
WARNING. Note that the item of the collection and the accumulator come in different order in inject and each_with_object.
NOTE: Defined in active_support/core_ext/enumerable.rb.
The method index_by generates a hash with the elements of an enumerable indexed by some key.
It iterates through the collection and passes each element to a block. The element will be keyed by the value returned by the block:
invoices.index_by(&:number)
- => {’2009-032’ => , ‘2009-008’ => , …}
WARNING. Keys should normally be unique. If the block returns the same value for different elements no collection is built for that key. The last item will win.
NOTE: Defined in active_support/core_ext/enumerable.rb.
The method many? is shorthand for collection.size > 1:
<% if pages.many? >
<= pagination_links >
< end %>
If an optional block is given many? only takes into account those elements that return true:
@see_more = videos.many? {|video| video.category == params[:category]}
NOTE: Defined in active_support/core_ext/enumerable.rb.
The predicate exclude? tests whether a given object does not belong to the collection. It is the negation of the builtin include?:
to_visit << node if visited.exclude?(node)
NOTE: Defined in active_support/core_ext/enumerable.rb.
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:
%w(a b c d).to(2) # => %w(a b c)
[].to(7) # => []
Similarly, from returns the tail from the element at the passed index on:
%w(a b c d).from(2) # => %w(c d)
%w(a b c d).from(10) # => nil
[].from(0) # => []
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:
shape_type = [Circle, Square, Triangle].rand
NOTE: Defined in active_support/core_ext/array/access.rb.
When the last argument in a method call is a hash, except perhaps for a &block argument, Ruby allows you to omit the brackets:
User.exists?(:email => params[:email])
That syntactic sugar is used a lot in Rails to avoid positional arguments where there would be too many, offering instead interfaces that emulate named parameters. In particular it is very idiomatic to use a trailing hash for options.
If a method expects a variable number of arguments and uses * in its declaration, however, such an options hash ends up being an item of the array of arguments, where kind of loses its role.
In those cases, you may give an options hash a distinguished treatment with extract_options!. That method checks the type of the last item of an array. If it is a hash it pops it and returns it, otherwise returns an empty hash.
Let’s see for example the definition of the caches_action controller macro:
def caches_action(*actions)
return unless cache_configured?
options = actions.extract_options!
…
end
This method receives an arbitrary number of action names, and an optional hash of options as last argument. With the call to extract_options! you obtain the options hash and remove it from actions in a simple and explicit way.
NOTE: Defined in active_support/core_ext/array/extract_options.rb.
The method to_sentence turns an array into a string containing a sentence that enumerates its items:
%w().to_sentence # => ""
%w(Earth).to_sentence # => “Earth”
%w(Earth Wind).to_sentence # => “Earth and Wind”
%w(Earth Wind Fire).to_sentence # => “Earth, Wind, and Fire”
This method accepts three options:
- :two_words_connector: What is used for arrays of length 2. Default is " and ".
- :words_connector: What is used to join the elements of arrays with 3 or more elements, except for the last two. Default is ", ".
- :last_word_connector: What is used to join the last items of an array with 3 or more elements. Default is ", and ".
The defaults for these options can be localised, their keys are:
| Option | I18n key |
|---|---|
| :two_words_connector | support.array.two_words_connector |
| :words_connector | support.array.words_connector |
| :last_word_connector | support.array.last_word_connector |
Options :connector and :skip_last_comma are deprecated.
NOTE: Defined in active_support/core_ext/array/conversions.rb.
The method to_formatted_s acts like to_s by default.
If the array contains items that respond to id, however, it may be passed the symbol :db as argument. That’s typically used with collections of ARs, though technically any object in Ruby 1.8 responds to id indeed. Returned strings are:
[].to_formatted_s(:db) # => “null”
[user].to_formatted_s(:db) # => “8456”
invoice.lines.to_formatted_s(:db) # => “23,567,556,12”
Integers in the example above are supposed to come from the respective calls to id.
NOTE: Defined in active_support/core_ext/array/conversions.rb.
The method to_xml returns a string containing an XML representation of its receiver:
Contributor.all(:limit => 2, :order => ‘rank ASC’).to_xml
- =>
- <?xml version=“1.0” encoding=“UTF-8”?>
- 4356
- Jeremy Kemper
- 1
- jeremy-kemper
- 4404
- David Heinemeier Hansson
- 2
- david-heinemeier-hansson
To do so it sends to_xml to every item in turn, and collects the results under a root node. All items must respond to to_xml, an exception is raised otherwise.
By default, the name of the root element is the underscorized and dasherized plural of the name of the class of the first item, provided the rest of elements belong to that type (checked with is_a?) and they are not hashes. In the example above that’s “contributors”.
If there’s any element that does not belong to the type of the first one the root node becomes “records”:
[Contributor.first, Commit.first].to_xml
- =>
- <?xml version=“1.0” encoding=“UTF-8”?>
- 4583
- Aaron Batalion
- 53
- aaron-batalion
- Joshua Peek
- 2009-09-02T16:44:36Z
- origin/master
- 2009-09-02T16:44:36Z
- Joshua Peek
- 190316
- false
- Kill AMo observing wrap_with_notifications since ARes was only using it
- 723a47bfb3708f968821bc969a9a3fc873a3ed58
If the receiver is an array of hashes the root element is by default also “records”:
[{:a => 1, :b => 2}, {:c => 3}].to_xml
- =>
- <?xml version=“1.0” encoding=“UTF-8”?>
- 2
- 1
- 3
WARNING. If the collection is empty the root element is by default “nil-classes”. That’s a gotcha, for example the root element of the list of contributors above would not be “contributors” if the collection was empty, but “nil-classes”. You may use the :root option to ensure a consistent root element.
The name of children nodes is by default the name of the root node singularized. In the examples above we’ve seen “contributor” and “record”. The option :children allows you to set these node names.
The default XML builder is a fresh instance of Builder::XmlMarkup. You can configure your own builder via the :builder option. The method also accepts options like :dasherize and friends, they are forwarded to the builder:
Contributor.all(:limit => 2, :order => ‘rank ASC’).to_xml(:skip_types => true)
- =>
- <?xml version=“1.0” encoding=“UTF-8”?>
- 4356
- Jeremy Kemper
- 1
- jeremy-kemper
- 4404
- David Heinemeier Hansson
- 2
- david-heinemeier-hansson
NOTE: Defined in active_support/core_ext/array/conversions.rb.
The class method Array.wrap behaves like the function Array() except that it does not try to call to_a on its argument. That changes the behaviour for enumerables:
Array.wrap(:foo => :bar) # => [{:foo => :bar}]
Array(:foo => :bar) # => [[:foo, :bar]]
Array.wrap(“foo\nbar”) # => [“foo\nbar”]
Array(“foo\nbar”) # => [“foo\n”, “bar”], in Ruby 1.8
NOTE: Defined in active_support/core_ext/array/wrap.rb.
The method in_groups_of splits an array into consecutive groups of a certain size. It returns an array with the groups:
[1, 2, 3].in_groups_of(2) # => [[1, 2], [3, nil]]
or yields them in turn if a block is passed:
<% sample.in_groups_of(3) do |a, b, c| >
<=h a >
<=h b >
<=h c %>
<% end %>
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:
[1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]
And you can tell the method not to fill the last group passing false:
[1, 2, 3].in_groups_of(2, false) # => [[1, 2], 3]
As a consequence false can’t be a used as a padding value.
NOTE: Defined in active_support/core_ext/array/grouping.rb.
The method in_groups splits an array into a certain number of groups. The method returns and array with the groups:
%w(1 2 3 4 5 6 7).in_groups(3)
- => [[“1”, “2”, “3”], [“4”, “5”, nil], [“6”, “7”, nil]]
or yields them in turn if a block is passed:
%w(1 2 3 4 5 6 7).in_groups(3) {|group| p group}
[“1”, “2”, “3”]
[“4”, “5”, nil]
[“6”, “7”, nil]
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:
%w(1 2 3 4 5 6 7).in_groups(3, “0”)
- => [[“1”, “2”, “3”], [“4”, “5”, “0”], [“6”, “7”, “0”]]
And you can tell the method not to fill the smaller groups passing false:
%w(1 2 3 4 5 6 7).in_groups(3, false)
- => [[“1”, “2”, “3”], [“4”, “5”], [“6”, “7”]]
As a consequence false can’t be a used as a padding value.
NOTE: Defined in active_support/core_ext/array/grouping.rb.
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:
(-5..5).to_a.split { |i| i.multiple_of?(4) }
- => [[-5], [-3, -2, -1], [1, 2, 3], 5]
Otherwise, the value received as argument, which defaults to nil, is the separator:
[0, 1, -5, 1, 1, “foo”, “bar”].split(1)
- => [0, [-5], [], [“foo”, “bar”]]
TIP: Observe in the previous example that consecutive separators result in empty arrays.
NOTE: Defined in active_support/core_ext/array/grouping.rb.
The method to_xml returns a string containing an XML representation of its receiver:
{"foo" => 1, “bar” => 2}.to_xml
- =>
- <?xml version=“1.0” encoding=“UTF-8”?>
- 1
- 2
To do so, the method loops over the pairs and builds nodes that depend on the values. Given a pair key, value:
- If value is a hash there’s a recursive call with key as :root.
- If value is an array there’s a recursive call with key as :root, and key singularized as :children.
- If value is a callable object it must expect one or two arguments. Depending on the arity, the callable is invoked with the options hash as first argument with key as :root, and key singularized as second argument. Its return value becomes a new node.
- If value responds to to_xml the method is invoked with key as :root.
- Otherwise, a node with key as tag is created with a string representation of value as text node. If value is nil an attribute “nil” set to “true” is added. Unless the option :skip_types exists and is true, an attribute “type” is added as well according to the following mapping:
XML_TYPE_NAMES = {
“Symbol” => “symbol”,
“Fixnum” => “integer”,
“Bignum” => “integer”,
“BigDecimal” => “decimal”,
“Float” => “float”,
“TrueClass” => “boolean”,
“FalseClass” => “boolean”,
“Date” => “date”,
“DateTime” => “datetime”,
“Time” => “datetime”
}
By default the root node is “hash”, but that’s configurable via the :root option.
The default XML builder is a fresh instance of Builder::XmlMarkup. You can configure your own builder with the :builder option. The method also accepts options like :dasherize and friends, they are forwarded to the builder.
NOTE: Defined in active_support/core_ext/hash/conversions.rb.
Ruby has a builtin method Hash#merge that merges two hashes:
{:a => 1, :b => 1}.merge(:a => 0, :c => 2)
- => {:a => 0, :b => 1, :c => 2}
Active Support defines a few more ways of merging hashes that may be convenient.
In case of collision the key in the hash of the argument wins in merge. You can support option hashes with default values in a compact way with this idiom:
options = {:length => 30, :omission => “…”}.merge(options)
Active Support defines reverse_merge in case you prefer this alternative notation:
options = options.reverse_merge(:length => 30, :omission => “…”)
And a bang version reverse_merge! that performs the merge in place:
options.reverse_merge!(:length => 30, :omission => “…”)
WARNING. Take into account that reverse_merge! may change the hash in the caller, which may or may not be a good idea.
NOTE: Defined in active_support/core_ext/hash/reverse_merge.rb.
The method reverse_update is an alias for reverse_merge!, explained above.
WARNING. Note that reverse_update has no bang.
NOTE: Defined in active_support/core_ext/hash/reverse_merge.rb.
As you can see in the previous example if a key is found in both hashes the value in the one in the argument wins.
Active Support defines Hash#deep_merge. In a deep merge, if a key is found in both hashes and their values are hashes in turn, then their merge becomes the value in the resulting hash:
{:a => {:b => 1}}.deep_merge(:a => {:c => 2})
- => {:a => {:b => 1, :c => 2}}
The method deep_merge! performs a deep merge in place.
NOTE: Defined in active_support/core_ext/hash/deep_merge.rb.
The method diff returns a hash that represents a diff of the receiver and the argument with the following logic:
- Pairs key, value that exist in both hashes do not belong to the diff hash.
- If both hashes have key, but with different values, the pair in the receiver wins.
- The rest is just merged.
{:a => 1}.diff(:a => 1)
- => {}, first rule
{:a => 1}.diff(:a => 2)
- => {:a => 1}, second rule
{:a => 1}.diff(:b => 2)
- => {:a => 1, :b => 2}, third rule
{:a => 1, :b => 2, :c => 3}.diff(:b => 1, :c => 3, :d => 4)
- => {:a => 1, :b => 2, :d => 4}, all rules
{}.diff({}) # => {}
{:a => 1}.diff({}) # => {:a => 1}
{}.diff(:a => 1) # => {:a => 1}
An important property of this diff hash is that you can retrieve the original hash by applying diff twice:
hash.diff(hash2).diff(hash2) == hash
Diffing hashes may be useful for error messages related to expected option hashes for example.
NOTE: Defined in active_support/core_ext/hash/diff.rb.
The method except returns a hash with the keys in the argument list removed, if present:
{:a => 1, :b => 2}.except(:a) # => {:b => 2}
If the receiver responds to convert_key, the method is called on each of the arguments. This allows except to play nice with hashes with indifferent access for instance:
{:a => 1}.with_indifferent_access.except(:a) # => {}
{:a => 1}.with_indifferent_access.except(“a”) # => {}
The method except may come in handy for example when you want to protect some parameter that can’t be globally protected with attr_protected:
params[:account] = params[:account].except(:plan_id) unless admin?
@account.update_attributes(params[:account])
There’s also the bang variant except! that removes keys in the very receiver.
NOTE: Defined in active_support/core_ext/hash/except.rb.
The method stringify_keys returns a hash that has a stringified version of the keys in the receiver. It does so by sending to_s to them:
{nil => nil, 1 => 1, :a => :a}.stringify_keys
- => {"" => nil, “a” => :a, “1” => 1}
The result in case of collision is undefined:
{"a" => 1, :a => 2}.stringify_keys
- => {"a" => 2}, in my test, can’t rely on this result though
This method may be useful for example to easily accept both symbols and strings as options. For instance ActionView::Helpers::FormHelper defines:
def to_check_box_tag(options = {}, checked_value = “1”, unchecked_value = “0”)
options = options.stringify_keys
options[“type”] = “checkbox”
…
end
The second line can safely access the “type” key, and let the user to pass either :type or “type”.
There’s also the bang variant stringify_keys! that stringifies keys in the very receiver.
NOTE: Defined in active_support/core_ext/hash/keys.rb.
The method symbolize_keys returns a hash that has a symbolized version of the keys in the receiver, where possible. It does so by sending to_sym to them:
{nil => nil, 1 => 1, “a” => "a"}.symbolize_keys
- => {1 => 1, nil => nil, :a => "a"}
WARNING. Note in the previous example only one key was symbolized.
The result in case of collision is undefined:
{"a" => 1, :a => 2}.symbolize_keys
- => {:a => 2}, in my test, can’t rely on this result though
This method may be useful for example to easily accept both symbols and strings as options. For instance ActionController::UrlRewriter defines
def rewrite_path(options)
options = options.symbolize_keys
options.update(options[:params].symbolize_keys) if options[:params]
…
end
The second line can safely access the :params key, and let the user to pass either :params or “params”.
There’s also the bang variant symbolize_keys! that symbolizes keys in the very receiver.
NOTE: Defined in active_support/core_ext/hash/keys.rb.
The methods to_options and to_options! are respectively aliases of symbolize_keys and symbolize_keys!.
NOTE: Defined in active_support/core_ext/hash/keys.rb.
The method assert_valid_keys receives an arbitrary number of arguments, and checks whether the receiver has any key outside that white list. If it does ArgumentError is raised.
{:a => 1}.assert_valid_keys(:a) # passes
{:a => 1}.assert_valid_keys(“a”) # ArgumentError
Active Record does not accept unknown options when building associations for example. It implements that control via assert_valid_keys:
mattr_accessor :valid_keys_for_has_many_association
@@valid_keys_for_has_many_association = [
:class_name, :table_name, :foreign_key, :primary_key,
:dependent,
:select, :conditions, :include, :order, :group, :having, :limit, :offset,
:as, :through, :source, :source_type,
:uniq,
:finder_sql, :counter_sql,
:before_add, :after_add, :before_remove, :after_remove,
:extend, :readonly,
:validate, :inverse_of
]
def create_has_many_reflection(association_id, options, &extension)
options.assert_valid_keys(valid_keys_for_has_many_association)
…
end
NOTE: Defined in active_support/core_ext/hash/keys.rb.
Ruby has builtin support for taking slices out of strings and arrays. Active Support extends slicing to hashes:
{:a => 1, :b => 2, :c => 3}.slice(:a, :c)
- => {:c => 3, :a => 1}
{:a => 1, :b => 2, :c => 3}.slice(:b, :X)
- => {:b => 2} # non-existing keys are ignored
If the receiver responds to convert_key keys are normalized:
{:a => 1, :b => 2}.with_indifferent_access.slice(“a”)
- => {:a => 1}
NOTE. Slicing may come in handy for sanitizing option hashes with a white list of keys.
There’s also slice! which in addition to perform a slice in place returns what’s removed:
hash = {:a => 1, :b => 2}
rest = hash.slice!(:a) # => {:b => 2}
hash # => {:a => 1}
NOTE: Defined in active_support/core_ext/hash/slice.rb.
The method with_indifferent_access returns an ActiveSupport::HashWithIndifferentAccess out of its receiver:
{:a => 1}.with_indifferent_access[“a”] # => 1
NOTE: Defined in active_support/core_ext/hash/indifferent_access.rb.
The method multiline? says whether a regexp has the /m flag set, that is, whether the dot matches newlines.
%r{.}.multiline? # => false
%r{.}m.multiline? # => true
Regexp.new(‘.’).multiline? # => false
Regexp.new(‘.’, Regexp::MULTILINE).multiline? # => true
Rails uses this method in a single place, also in the routing code. Multiline regexps are disallowed for route requirements and this flag eases enforcing that constraint.
def assign_route_options(segments, defaults, requirements)
…
if requirement.multiline?
raise ArgumentError, “Regexp multiline option not allowed in routing requirements: #{requirement.inspect}”
end
…
end
NOTE: Defined in active_support/core_ext/regexp.rb.
Active Support extends the method Range#to_s so that it understands an optional format argument. As of this writing the only supported non-default format is :db:
(Date.today..Date.tomorrow).to_s
- => “2009-10-25..2009-10-26”
(Date.today..Date.tomorrow).to_s(:db)
- => “BETWEEN ‘2009-10-25’ AND ‘2009-10-26’”
As the example depicts, the :db format generates a BETWEEN SQL clause. That is used by Active Record in its support for range values in conditions.
NOTE: Defined in active_support/core_ext/range/conversions.rb.
Active Support extends the method Range#step so that it can be invoked without a block:
(1..10).step(2) # => [1, 3, 5, 7, 9]
As the example shows, in that case the method returns and array with the corresponding elements.
NOTE: Defined in active_support/core_ext/range/blockless_step.rb.
The method Range#include? says whether some value falls between the ends of a given instance:
(2..3).include?(Math::E) # => true
Active Support extends this method so that the argument may be another range in turn. In that case we test whether the ends of the argument range belong to the receiver themselves:
(1..10).include?(3..7) # => true
(1..10).include?(0..7) # => false
(1..10).include?(3..11) # => false
(1…9).include?(3..9) # => false
WARNING: The orginal Range#include? is still the one aliased to Range#===.
NOTE: Defined in active_support/core_ext/range/include_range.rb.
The method Range#overlaps? says whether any two given ranges have non-void intersection:
(1..10).overlaps?(7..11) # => true
(1..10).overlaps?(0..7) # => true
(1..10).overlaps?(11..27) # => false
NOTE: Defined in active_support/core_ext/range/overlaps.rb.
As you surely know Ruby has an UnboundMethod class whose instances are methods that belong to the limbo of methods without a self. The method Module#instance_method returns an unbound method for example:
Hash.instance_method(:delete) # => #<UnboundMethod: Hash#delete>
An unbound method is not callable as is, you need to bind it first to an object with bind:
clear = Hash.instance_method(:clear)
clear.bind({:a => 1}).call # => {}
Active Support defines Proc#bind with an analogous purpose:
Proc.new { size }.bind([]).call # => 0
As you see that’s callable and bound to the argument, the return value is indeed a Method.
NOTE: To do so Proc#bind actually creates a method under the hood. If you ever see a method with a weird name like __bind_1256598120_237302 in a stack trace you know now where it comes from.
Action Pack uses this trick in rescue_from for example, which accepts the name of a method and also a proc as callbacks for a given rescued exception. It has to call them in either case, so a bound method is returned by handler_for_rescue, thus simplifying the code in the caller:
def handler_for_rescue(exception)
, rescuer = Array(rescue_handlers).reverse.detect do |klassname, handler|
…
end
end
NOTE: Defined in active_support/core_ext/proc.rb.
…
…
…
…
…
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:
File.atomic_write(joined_asset_path) do |cache|
cache.write(join_asset_file_contents(asset_paths))
end
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.
NOTE: Defined in active_support/core_ext/file/atomic.rb.
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:
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
NOTE: Defined in active_support/core_ext/name_error.rb.
Rails hijacks LoadError.new to return a MissingSourceFile exception:
$ ruby e ‘require “nonexistent”’ nonexistent (LoadError)
…: no such file to load -
…
$ script/runner ‘require “nonexistent”’
…: no such file to load — nonexistent (MissingSourceFile)
…
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:
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
NOTE: Defined in active_support/core_ext/load_error.rb.
- April 18, 2009: Initial version by Xavier Noria