When using the MongoRecord framework changes to existing documents (with the save method) insert a new document into MongoDB rather than updating the existing document as would be expected.

Some code that demonstrates this issue is as follows:

class Test < MongoRecord::Base
fields :name, :data
collection_name 'test'
end

Test.delete_all({})

t1 = Test.new({:name => 'test', :data => 'first'})
t1.save
t1.data = 'second'
t1.save

t2 = Test.find(:first, :conditions => {:name => 'test'})
t2.data = 'third'
t2.save

results = Test.find(:all)
results.each do |row|
p row
end