Ho can I select specific array element by index defined in document?
For example I have following document:
{ "_id" : 1, "idx" : 1, "vals" : [ 1, 2 ] }
And I want to select vals element defined by idx index.
I have managed to select specific array element defined by literal:
> db.test.find({_id:1}, {vals:{$slice:[1, 1]}})
{ "_id" : 1, "idx" : 1, "vals" : [ 2 ] }
But how can I use idx field in $slice operator?
The optimal way to do this is in MongoDB 3.2 using the $arrayElemAt operator:
db.test.aggregate([
{ "$project": { "vals": { "$arrayElemAt": [ "$vals", "$idx" ] } } }
])
You can also use findOne if you use _id in your query criteria and to get idx value.
var idx = db.test.findOne({ "_id": 1 }).idx
db.test.find({ "_id": 1 }, { "vals": { "$slice": [ idx, 1 ]}})
With find you need to use cursor.map
db.test.find().map(function(doc) {
doc.vals = doc.vals.slice(doc.idx, 2);
return doc;
})
Result:
[ { "_id" : 1, "asd" : 1, "vals" : [ 2 ] } ]