instance method `class_attribute`

Ruby on Rails 3.0.20

Since v2.3.18

Available in: v2.3.18 v3.0.20 v3.1.12 v3.2.22.5 v4.0.13 v4.1.16 v4.2.9 v5.2.8.1 v6.0.6 v6.1.7.10 v7.0.10 v7.1.6 v7.2.3 v8.0.4 v8.1.2

Compare with:

class_attribute(*attrs)

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

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 places but 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, a query method is defined as well:

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

Parameters

attrs rest

Source

# File activesupport/lib/active_support/core_ext/class/attribute.rb, line 68
  def class_attribute(*attrs)
    options = attrs.extract_options!
    instance_reader = options.fetch(:instance_reader, true)
    instance_writer = options.fetch(:instance_writer, true)

    attrs.each do |name|
      class_eval <<-RUBY, __FILE__, __LINE__ + 1
        def self.#{name}() nil end
        def self.#{name}?() !!#{name} end

        def self.#{name}=(val)
          singleton_class.class_eval do
            remove_possible_method(:#{name})
            define_method(:#{name}) { val }
          end
          val
        end

        if instance_reader
          remove_possible_method :#{name}
          def #{name}
            defined?(@#{name}) ? @#{name} : singleton_class.#{name}
          end

          def #{name}?
            !!#{name}
          end
        end
      RUBY

      attr_writer name if instance_writer
    end
  end

Defined in activesupport/lib/active_support/core_ext/class/attribute.rb line 68 · View on GitHub · Improve this page · Find usages on GitHub

Defined in Class

Signature

Parameters