Exact match when searching in arrays of array in 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!

mnemosyn · Accepted Answer · 2015-04-15 21:39:06Z

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.

answered Apr 15, 2015 at 21:39

mnemosyn

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