Class

Methods

class_attribute
descendants

Instance Public methods

`class_attribute(*attrs, instance_accessor: true, instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil )`

Declare a class-level attribute whose value is inheritable by subclasses. Subclasses can change their own value and it will not impact parent class.

Options

:instance_reader - Sets the instance reader method (defaults to true).
:instance_writer - Sets the instance writer method (defaults to true).
:instance_accessor - Sets both instance methods (defaults to true).
:instance_predicate - Sets a predicate method (defaults to true).
:default - Sets a default value for the attribute (defaults to nil).

Examples

class Base
  class_attribute :setting
end

class Subclass < Base
end

Base.setting = true
Subclass.setting            # => true
Subclass.setting = false
Subclass.setting            # => false
Base.setting                # => true

In the above case as long as Subclass does not assign a value to setting by performing Subclass.setting = something, Subclass.setting would read value assigned to parent class. Once Subclass assigns a value then the value assigned by Subclass would be returned.

This matches normal Ruby method inheritance: think of writing an attribute on a subclass as overriding the reader method. However, you need to be aware when using class_attribute with mutable structures as Array or Hash. In such cases, you don’t want to do changes in place. Instead use setters:

Base.setting = []
Base.setting                # => []
Subclass.setting            # => []

# Appending in child changes both parent and child because it is the same object:
Subclass.setting << :foo
Base.setting               # => [:foo]
Subclass.setting           # => [:foo]

# Use setters to not propagate changes:
Base.setting = []
Subclass.setting += [:foo]
Base.setting               # => []
Subclass.setting           # => [:foo]

For convenience, an instance predicate method is defined as well. To skip it, pass instance_predicate: false.

Subclass.setting?       # => false

Instances may overwrite the class value in the same way:

Base.setting = true
object = Base.new
object.setting          # => true
object.setting = false
object.setting          # => false
Base.setting            # => true

To opt out of the instance reader method, pass instance_reader: false.

object.setting          # => NoMethodError
object.setting?         # => NoMethodError

To opt out of the instance writer method, pass instance_writer: false.

object.setting = false  # => NoMethodError

To opt out of both instance methods, pass instance_accessor: false.

To set a default value for the attribute, pass default:, like so:

class_attribute :settings, default: {}

📝 Source code

# File activesupport/lib/active_support/core_ext/class/attribute.rb, line 86
def class_attribute(*attrs, instance_accessor: true,
  instance_reader: instance_accessor, instance_writer: instance_accessor, instance_predicate: true, default: nil
)
  class_methods, methods = [], []
  attrs.each do |name|
    unless name.is_a?(Symbol) || name.is_a?(String)
      raise TypeError, "#{name.inspect} is not a symbol nor a string"
    end

    name = name.to_sym
    namespaced_name = :"__class_attr_#{name}"
    ::ActiveSupport::ClassAttribute.redefine(self, name, namespaced_name, default)

    class_methods << "def #{name}; #{namespaced_name}; end"
    class_methods << "def #{name}=(value); self.#{namespaced_name} = value; end"

    if singleton_class?
      methods << <<~RUBY if instance_reader
        silence_redefinition_of_method(:#{name})
        def #{name}
          self.singleton_class.#{name}
        end
      RUBY
    else
      methods << <<~RUBY if instance_reader
        silence_redefinition_of_method def #{name}
          if defined?(@#{name})
            @#{name}
          else
            self.class.#{name}
          end
        end
      RUBY
    end

    methods << <<~RUBY if instance_writer
      silence_redefinition_of_method(:#{name}=)
      attr_writer :#{name}
    RUBY

    if instance_predicate
      class_methods << "silence_redefinition_of_method def #{name}?; !!self.#{name}; end"
      if instance_reader
        methods << "silence_redefinition_of_method def #{name}?; !!self.#{name}; end"
      end
    end
  end

  location = caller_locations(1, 1).first
  class_eval(["class << self", *class_methods, "end", *methods].join(";").tr("\n", ";"), location.path, location.lineno)
end

🔎 See on GitHub

`descendants()`

Returns an array with all classes that are < than its receiver.

class C; end
C.descendants # => []

class B < C; end
C.descendants # => [B]

class A < B; end
C.descendants # => [B, A]

class D < C; end
C.descendants # => [B, A, D]