Gem Used Only For A Migration
Figured out how to do this, mostly by the insight here. Added a Gemfile just for the migration, included the project’s Gemfile with
instance_eval(File.read(File.join(File.dirname( __FILE__ ), '..', '..', 'Gemfile'))). When running the migration, just added in environment variable
BUNDLE_GEMFILE to use that migration specific Gemfile.