问题
I have a db set up in mongo that I'm accessing with pymongo.
I'd like to be able to pull a small set of fields into a list of dictionaries. So, something like what I get in the mongo shell when I type...
db.find({},{"variable1_of_interest":1, "variable2_of_interest":1}).limit(2).pretty()
I'd like a python statement like:
x = db.find({},{"variable1_of_interest":1, "variable2_of_interest":1})
where x is an array structure of some kind rather than a cursor---that is, instead of iterating, like:
data = []
x = db.find({},{"variable1_of_interest":1, "variable2_of_interest":1})
for i in x:
data.append(x)
Is it possible that I could use MapReduce to bring this into a one-liner? Something like
db.find({},{"variable1_of_interest":1, "variable2_of_interest":1}).map_reduce(mapper, reducer, "data")
I intend to output this dataset to R for some analysis, but I'd like concentrate the IO in Python.
回答1:
You don't need to call mapReduce, you just turn the cursor into a list like so:
>>> data = list(col.find({},{"a":1,"b":1,"_id":0}).limit(2))
>>> data
[{u'a': 1.0, u'b': 2.0}, {u'a': 2.0, u'b': 3.0}]
where col is your db.collection object.
But caution with large/huge result cause every thing is loaded into memory.
回答2:
What you can do is to call mapReduce in pymongo and pass it the find query as an argument, it could be like this:
db.yourcollection.Map_reduce(map_function, reduce_function,query='{}')
About the projections I think that you would need to do them in the reduce function since query only specify the selection criteria as it says in the mongo documentation
回答3:
Building off of Asya's answer: If you wanted a list of just one value in each entry as opposed to a list of objects--using a list comprehension worked for me.
I.e. if each object represents a user and the database stored their email, and you say wanted all the users that were 15 years old
user_emails = [user['email'] for user in db.people.find( {'age' : 15} )]
More here
来源：https://stackoverflow.com/questions/15092884/how-can-i-return-an-array-of-mongodb-objects-in-pymongo-without-a-cursor-can