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Editorial Team
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Asked: 2026-05-25T03:20:31+00:00

In our Rails app environment (ruby 1.8.7 / rails 2.3.12) I can run the

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In our Rails app environment (ruby 1.8.7 / rails 2.3.12) I can run the following code:

class MongoTester
  include MongoMapper::Document
  key :test_arr, Array, :default => []
end

mt = MongoTester.new
mt.test_arr << 24

mt2 = MongoTester.new

The output of mt2.inspect is:

 #<MongoTester test_arr: [24], _id: BSON::ObjectId('4e5c089f3beaacad00000002')>

I’m not sure how this is possible. object_id and _id for both records are different. Neither one is saved. We haven’t modified MongoMapper in our application (mongomapper 0.8.6). Yet a completely new record contains the data of a previously created different record.

I can’t reproduce this in MongoMapper’s test suite.

Any information on how this effect is possible or how I can get rid of it would be awesome. Thanks!

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1 Answer

  1. Editorial Team

    That is a bug: file it at https://github.com/jnunemaker/mongomapper/issues/new This bug is fixed in newer versions of MongoMapper

    Here’s a workaround:

    key :test_arr, Array, :default => lambda { [] }

    It’s because arrays in Ruby are mutable, so your default array object is getting added to each time with <<. The object_id’s of your docs will be different, but the object_id’s of the arrays will be the same.

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