Q:

MongoDB/PyMongo won't $set attribute to document - but sets all other attributes! ( bizarre error)

I'm trying to write a defaultdict variable to a document in my MongoDB.  Everything else sets fine, just not this one attribute, its bizarre!  I'm setting a rather large defaultdict called 'domains', which has worked many times before.  Check out this terminal output:
So here's my defaultdict:
>>> type(domains) 
<type 'collections.defaultdict'>

Its pretty big, about 3mb: 
>>> sys.getsizeof(domains)
3146008

Here's the document we'll set it to:
>>> db. AggregateResults.find_one({'date':'20110409'}).keys()
[u'res', u'date', u'_id']

Let's grab that document's ID:
>>> myID = db. AggregateResults.find_one({'date':'20110409'})['_id']
>>> myID
ObjectId('50870847f49a00509a000000')

Great, let's set the attribute:
>>> db. AggregateResults.update({'_id':myID}, {"$set": {'domains':domains}})
>>> db. AggregateResults.find_one({'date':'20110409'}).keys()
[u'res', u'date', u'_id']

EH?  It didn't save??  
Hmmm...does anything save at all? 
>>> db. AggregateResults.update({'_id':myID}, {"$set": {'myTest':'hello world'}})
>>> db. AggregateResults.find_one({'date':'20110409'}).keys()
[u'myTest', u'res', u'date', u'_id']

Okay...so it can save things fine...perhaps its because MongoDB doesn't like defaultdicts?  Let's try:
>>> myDD = defaultdict(int)
>>> myDD['test'] = 1
>>> myDD
defaultdict(<type 'int'>, {'test': 1})
>>> db. AggregateResults.update({'_id':myID}, {"$set": {'myDD':myDD}})
>>> db. AggregateResults.find_one({'date':'20110409'}).keys()
[u'myTest', u'res', u'date', u'myDD', u'_id']

So it can save defaultdicts fine, just not this one?? 
So strange!  Any ideas why?? 
EDIT with safe=True:
>>> db. AggregateResults.update({'_id':myID}, {"$set": {'domains':domains}}, safe=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.6/site-packages/pymongo-2.1.1_-py2.6-linux-x86_64.egg/pymongo/collection.py", line 405, in update
    _check_keys, self.__uuid_subtype), safe)
  File "/usr/lib64/python2.6/site-packages/pymongo-2.1.1_-py2.6-linux-x86_64.egg/pymongo/connection.py", line 796, in _send_message
    return self.__check_response_to_last_error(response)
  File "/usr/lib64/python2.6/site-packages/pymongo-2.1.1_-py2.6-linux-x86_64.egg/pymongo/connection.py", line 746, in __check_response_to_last_error
    raise OperationFailure(error["err"], error["code"])
pymongo.errors. OperationFailure: not okForStorage

This GoogleGroup discussion says that could be due to having fullstops in the keys, but:
>>> [x for x in domains.keys() if '.'  in x]
[]

A:

Aha!  Found it!  
Not only can keys in MongoDB not have '.',  they also cannot have '$' in them.  
See:
>>>[x for x in domains.keys() if '$' in x]
['$some_key_']

