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DeprecatedConstantAccessor transforms a constant into a deprecated one by hooking const_missing.

It takes the names of an old (deprecated) constant and of a new constant (both in string form) and optionally a deprecator. The deprecator defaults to ActiveSupport::Deprecator if none is specified.

The deprecated constant now returns the same object as the new one rather than a proxy object, so it can be used transparently in rescue blocks etc.

PLANETS = %w(mercury venus earth mars jupiter saturn uranus neptune pluto)

# (In a later update, the original implementation of `PLANETS` has been removed.)

PLANETS_POST_2006 = %w(mercury venus earth mars jupiter saturn uranus neptune)
include ActiveSupport::Deprecation::DeprecatedConstantAccessor
deprecate_constant 'PLANETS', 'PLANETS_POST_2006'

PLANETS.map { |planet| planet.capitalize }
# => DEPRECATION WARNING: PLANETS is deprecated! Use PLANETS_POST_2006 instead.
     (Backtrace information…)
     ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
Methods
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Class Public methods
included(base)
# File activesupport/lib/active_support/deprecation/constant_accessor.rb, line 28
def self.included(base)
  require "active_support/inflector/methods"

  extension = Module.new do
    def const_missing(missing_const_name)
      if class_variable_defined?(:@@_deprecated_constants)
        if (replacement = class_variable_get(:@@_deprecated_constants)[missing_const_name.to_s])
          replacement[:deprecator].warn(replacement[:message] || "#{name}::#{missing_const_name} is deprecated! Use #{replacement[:new]} instead.", caller_locations)
          return ActiveSupport::Inflector.constantize(replacement[:new].to_s)
        end
      end
      super
    end

    def deprecate_constant(const_name, new_constant, message: nil, deprecator: ActiveSupport::Deprecation.instance)
      class_variable_set(:@@_deprecated_constants, {}) unless class_variable_defined?(:@@_deprecated_constants)
      class_variable_get(:@@_deprecated_constants)[const_name.to_s] = { new: new_constant, message: message, deprecator: deprecator }
    end
  end
  base.singleton_class.prepend extension
end
Instance Public methods
const_missing(missing_const_name)
# File activesupport/lib/active_support/deprecation/constant_accessor.rb, line 32
def const_missing(missing_const_name)
  if class_variable_defined?(:@@_deprecated_constants)
    if (replacement = class_variable_get(:@@_deprecated_constants)[missing_const_name.to_s])
      replacement[:deprecator].warn(replacement[:message] || "#{name}::#{missing_const_name} is deprecated! Use #{replacement[:new]} instead.", caller_locations)
      return ActiveSupport::Inflector.constantize(replacement[:new].to_s)
    end
  end
  super
end
deprecate_constant(const_name, new_constant, message: nil, deprecator: ActiveSupport::Deprecation.instance)
# File activesupport/lib/active_support/deprecation/constant_accessor.rb, line 42
def deprecate_constant(const_name, new_constant, message: nil, deprecator: ActiveSupport::Deprecation.instance)
  class_variable_set(:@@_deprecated_constants, {}) unless class_variable_defined?(:@@_deprecated_constants)
  class_variable_get(:@@_deprecated_constants)[const_name.to_s] = { new: new_constant, message: message, deprecator: deprecator }
end