/usr/lib/ruby/vendor_ruby/nulldb/rails.rb is in ruby-activerecord-nulldb-adapter 0.3.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | require 'nulldb/core'
# Need to defer calling Rails.root because when bundler loads, Rails.root is nil
NullDB.configure {|ndb| def ndb.project_root;Rails.root;end}
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