gistfile1.txt

Using MongoMapper 0.5.4 with mongo-0.15.1 and mongo_ext-0.15.1

require 'rubygems'
require 'mongo_mapper'

MongoMapper.connection = Mongo::Connection.new('localhost', 27017)
MongoMapper.database = 'dupes'

class Doc
  include MongoMapper::Document
  
  key :data, String
end

Doc.delete_all

d = Doc.new :data => "First doc"
d.save

d = Doc.new :name => "Second doc (not using defined key)"
d.save

d = Doc.new '_id' => "1234534343433", :data => "Doc with preset id"
d.save

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Results:
> db.docs.find()
{ "_id" : "4ae09ac962125b23dc000001", "_id" : "4ae09ac962125b23dc000001", "data" : "First doc" }
{ "_id" : "4ae09ac962125b23dc000002", "_id" : "4ae09ac962125b23dc000002", "name" : "Second doc (not using defined key)" }
{ "_id" : "1234534343433", "_id" : "1234534343433", "data" : "Doc with preset id" }


-------
With mongo_ext uninstalled:
> db.docs.find()
{ "_id" : "4ae09c5062125b2432000001", "data" : "First doc" }
{ "_id" : "4ae09c5062125b2432000002", "name" : "Second doc (not using defined key)" }
{ "_id" : "1234534343433", "data" : "Doc with preset id" }