In MongoDB how to return only part of array?
mongodb, pymongo
Solution
Have you tried adding another inclusion projection? I think you may be able to add something trivial like foo:1 (that is not a real field) and it should work.
Like so:
{ 'countries' : { '$slice' : [0, 3] }, '_id' : 0, foo : 1 }
If it doesn't work I suggest filing a bug with mongo. They are actually very good about responding to bugs.
Problem
Consider collection "fruits", in which I have this document (I'm using Python's pymongo driver, btw): ``` { '_id' : 'lemons', 'weight' : 58, 'shape' : 'oval', 'countries' : ['Mexico', 'Turkey', 'Argentina', 'SAfrica', 'US'] } ``` Now, if I want to get only the 'countries' field, this query works just fine: ``` In [1]: find_one('lemons', { 'countries' : 1, '_id' : 0 }) Out[1]: {u'countries': [u'Mexico', u'Turkey', u'Argentina', u'SAfrica', u'US']} ``` But it turns out that what I really need is just list of few top-countries, not all of them, so I'm using "$slice" instead of plain True/1: ``` In [239]: c.find_one('lemons', { 'countries' : { '$slice' : [0, 3] }, '_id' : 0 }) Out[239]: {u'countries': [u'Mexico', u'Turkey', u'Argentina'], u'shape': u'oval', u'weight': 58} ``` Well, number of countries has shrinked, but now it gives me whole lot of other unrelated information! Q: Is there any way to show only those fields that I have asked for? Additionally listing '_id' as exception is fine, because this field is always presented, but I can't be sure about other fields, since MongoDB is scheme-less and I intend to use this feature to add additional fields if needed.