https://github.com/mongodb/mongo-ruby-driver/blob/1.x-stable/lib/mongo/utils/core_ext.rb#L30

Doesn't check if the function already exists, so it overrides rails' implementation ( see https://github.com/rails/rails/blob/4-2-stable/activesupport/lib/active_support/core_ext/hash/keys.rb#L71 ) with a slightly different implementation, which causes compatibilities issues on seemingly unrelated topics (see https://github.com/Casecommons/pg_search/issues/257 )