jon = User.create(first_name: 'jon', last_name: 'kern', email: 'jo...@example.com', organization: umfc)
NoMethodError: undefined method `exists?' for nil:NilClass/Users/jon/.rvm/gems/ruby-2.1.1@blazemark4/bundler/gems/mongomapper-9b87dee50780/lib/mongo_mapper/plugins/validations.rb:51:in `validate_each'
if @klass.exists?(conditions)
record.errors.add(attribute, :taken, options.except(:case_sensitive, :scope).merge(:value => value))
end