mongodb map/reduce or group
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I have a mongodb data with candidate details 

a property is qualification which is list based

doc1:  "qualification" : ["BS","Diploma"]

doc1:  "qualification" : ["BS","MS"]

doc1:  "qualification" : ["PG"]

doc1:  "qualification" : ["Diploma"]

doc1:  "qualification" : ["BS"]

doc1:  "qualification" : ["MS"]

doc1:  "qualification" : ["BS","MS","Phd"]

from this i generated top 3 qualification using mongodb map/reduce which calculates occurrences of each qualification and i filtered top 3

result is
{"BS":4,"MS":3,"Diploma":2}

but this map/reduce creates a temp collection every time, so i think it slows down my application, Can we use group statement for this scenario?
or as i am using python can i generate a list of all qualification to use counter()

which is feasible and less time consuming?

Thanks 
 