module Rails::Generators

Rails::Generators.fallbacks[:shoulda] = :test_unit

some of them are not available by adding a fallback:
Shoulda then can tell generators to search for test_unit generators when

test_unit ones.
of test_unit. However, most part of shoulda generators are similar to
For example, shoulda is considered a test_framework and is an extension

they can add a fallback.
generator group to fallback to another group in case of missing generators,
Hold configured generators fallbacks. If a plugin developer wants a

def fallbacks
  @fallbacks ||= {}
end