mongodb - project element in array

Question

a doc {"m":[1,2,3], "others":xxx}, get the first element in array 'm' by:

db.find({query},{"m":{$slice:1}, "m":1})

the return is {"m":[1]}, the element in doc is an array. But in this query, only one element in array will be get, so I don't need the return doc contain array which has only one element. This is like SQL local variable in sub-query. The element I want has no name in original doc, if I want get it then I need to make a name for it, the doc returned I wanted is like: {"localVariable":1} rather than {"m":[1]}

I try to project out the first element by:

db.find({query},{"m":{$slice:1}, "m.1":1})

but this don't work.

@jean I am guessing at your meaning. Do you want: "The first element of the array, but not as an array value, and just return the value? You cannot do that. Not with find. Maybe with aggregate but I still doubt this is what you want really. Just learn to use the array return value. It's an array with one element. That should not be hard. The way you have presented this question is clearly not your real coding use case. You would do better to ask a question that is about the real code and problem you need to solve. — Neil Lunn
– Neil Lunn, Commented Mar 15, 2014 at 5:00
From js api it indeed hard to distinguish the difference. I am using c++ driver, in this situation, the code writing is very boring. Every time I need get the m doc and convert it to a container of doc, and get the first element then convert it to int value. — jean
– jean, Commented Mar 15, 2014 at 6:05

Neil Lunn · Accepted Answer · 2014-03-15 03:44:43Z

1

You seem to be looking for a positional $ operator match. It works like this:

 db.collection.find({ m: 2 },{ "m.$": 1 })

That will return you the matched element

If you are in fact looking to always get the second element, then you need the two argument form of $slice:

 db.collection.find({ },{ "m": { "$slice": [1,1] } })

Tested output:

 db.test.insert({ m: [1,2,3] })
 db.test.find({ },{ "m": {$slice: [1,1]} })

  { "_id" : ObjectId("5323cc2770fde63cf1146ba3"), "m" : [  2 ] }

edited Mar 15, 2014 at 3:44

answered Mar 14, 2014 at 10:08

Neil Lunn

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3 Comments

jean Over a year ago

the return still be {"m":[1]}

Neil Lunn Over a year ago

@jean No it does not. See the output from what I have run added to the answer

jean Over a year ago

I revised my question. I want it return {"othername":1} rather than {"m":[1]}. I want project out the element which in array

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