soap4r + rails can't find SOAP::Mapping::EncodedRegistry

Joining a new project, I had to go through the usual setting-up rigmarole - which means finding several new things to break/configure/reinstall. One of which was to get soap4r to run.

I'd run gem install soap4r just fine, but running rake was giving me the error below:

rake aborted!
uninitialized constant SOAP::Mapping::EncodedRegistry

Scrounging around gave several solutions that didn't work, mainly using two of the three lines below. I required all three of the following before it ran for me.

# Add these lines to config/environment.rb just before the Rails initializer code
require 'rubygems'
gem 'soap4r'
require 'soap/mapping'