module DeprecatedConstantAccessor

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"]