在python中迭代mongo文档

Question

Please take a look at the following code:

collection = db_name.get_db().collection_name
print collection
# prints .. Collection(Database(Connection('localhost', 27017), u'db_name'), u'colelction_name')


for key in some_dict.keys():
        query = {"p_id":key}
        document = collection.find(query)
        print document
        # gives <pymongo.cursor.Cursor object at 0x7f13f3049b10>

Now I want to retreive this document.. and fetch the data. but if i do:

       for d in document:
            print d

I get the following error

    File "/usr/local/lib/python2.7/dist-packages/pymongo/cursor.py", line 703, in next
   if len(self.__data) or self._refresh():
 File "/usr/local/lib/python2.7/dist-packages/pymongo/cursor.py", line 666, in _refresh
self.__uuid_subtype))
  File "/usr/local/lib/python2.7/dist-packages/pymongo/cursor.py", line 628, in __send_message
self.__tz_aware)
  File "/usr/local/lib/python2.7/dist-packages/pymongo/helpers.py", line 101, in _unpack_response
error_object["$err"])

What am i doing wrong? Thanks

Answer 1

You want to retrieve "this document" - then use the official method for fetching one document matching your criteria: find_one():

http://api.mongodb.org/python/2.2/api/pymongo/collection.html

Reading basic API documentation is your friend.

Answer 2

Also, note that pymongo has several constructor options availble to a cursor :

https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/cursor.py

Some of these options (await_data, exhaust, for example) will make a big difference in terms of speeding cursor iteration.

Depending on the length and complexity of your query, you can also process data in a separate thread, so that you run through the cursor as fast as possible, firing of asynchronous tasks along the way.

I've found that running a single thread for exhausting a mongo cursor, with separate threads for processing data from it can increase cursor throughput drastically, regardless of the amount of data the cursor brings down with it.

Answer 3

I don't understand why you are trying to get a batch of documents using this approach, when the API and mongo provides queries to suit this?

If some_dict.keys() returns a list of id's, and you want to retrieve the documents matching those id's, why not use a proper "in" query?

docs = collection.find({'p_id': {'$in':some_dict.keys()}})
print docs
# <pymongo.cursor.Cursor object at 0x10112dfd0>
print [d for d in docs]
# [{DOC1}, {DOC2}, {...}]

As @ich recommended, the pymongo api docs explain everything, as well as reading the mongodb query language.

If this is not what you were looking for, then your intent was not clear in your question. It does simply look like you want to get a batch of documents matching a list of id's