CODEWITHSUNDEEP
mongodb
jean
jean

Reputation: 2990

mongodb - project element in array

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.

Upvotes: 5

Views: 2321

Answers (1)

Neil Lunn
Neil Lunn

Reputation: 151190

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 ] }

Upvotes: 1

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