Exact match when searching in arrays of array in MongoDB - mongodb

Question

I have two questions. I found similar things but I couldn't adapt to my problem.
query = {'$and': [{'cpc.class': u'24'},
{'cpc.section': u'A'},
{'cpc.subclass': u'C'}]}
collection:
{"_id":1,
"cpc":
[{u'class': u'24',
u'section': u'A',
u'subclass': u'B'},
{u'class': u'07',
u'section': u'C',
u'subclass': u'C'},]}
{"_id":2,
"cpc":
[{u'class': u'24',
u'section': u'A',
u'subclass': u'C'},
{u'class': u'07',
u'section': u'K',
u'subclass': u'L'},]}
In this query, two documents will be fetched.
1) But I want to fetch only the second document ("_id": 2) because it matches the query exactly. That is, the second document contains a cpc element which its class equals to 24, its section equals to A, and its subclass equals to C.
2) And I want to fetch only the matching element of cpc if possible? Otherwise I have to traverse all elements of each retrieved documents; if I traverse and try to find out which element matches exactly then my first question would be meaningless.
Thanks!

Answer 1

1) you're looking for the $elemMatch operator which compares subdocuments as a whole and is more concise then separate subelement queries (you don't need the $and in your query by the way):
query = { 'cpc' : {
'$elemMatch': { 'class': u'24',
'section': u'A',
'subclass': u'C' } } };
2) That can be done using a projection:
db.find(query, { "cpc.$" : 1 })
The $ projection operator documentation contains pretty much this use case as an example.