如何使用 Pymongo 和 MongoDB 返回特定字段？

Question

I am trying to return just two fields from my MongoDB database and this query correct sends back my username and posts from the client:

db.users.find({'email_address': /^johnny/}, {'username': 1, 'user_posts': 1, '_id': 0});

I'm trying to switch this to PyMongo so I can query in Python and my query looks like this:

regx = Regex('^{0}'.format('johnny'))
query = {'postcode': regx}, {'username': 1, 'user_posts': 1, '_id': 0}
user_list = mycol.find(query).limit(5)

The bit where it's failing is here:

{'username': 1, 'user_posts': 1, '_id': 0}

As without this filter the documents are sent in full fine. With that I get this error message from PyMongo:

TypeError: filter must be an instance of dict, bson.son.SON, or any other type that inherits from collections.Mapping

I assume that my filter is malformed and it's no longer a dictionary so have tried various alternatives of wrapping in quotes and then.format() etc but cannot seem to hit the magical combination.

Answer 1

Turned out it was quite easy.. I just needed to have two dictionaries. 1 for the query and the other for the filter.

regx = Regex('^{0}'.format('johnny'))
filter_stuff = {'username': 1, 'user_posts': 1, '_id': 0}

Then just build the full query as follows:

user_list = mycol.find(query, filter_stuff).limit(5)

Answer 2

The problem is that the following is not a dictionary but a tuple of two dictionaries:

query = {'postcode': regx}, {'username': 1, 'user_posts': 1, '_id': 0}

if you wanted to use it directly you could do it with

user_list = mycol.find(query[0], query[1]).limit(5)

but probably best to actually assign it to two separate variables like so

query, filter = {'postcode': regx}, {'username': 1, 'user_posts': 1, '_id': 0}

and then

user_list = mycol.find(query, filter).limit(5)