Get n-th element of an array in MongoDB

Question

As part of my document in MongoDB I'm storing an array of objects. How can I query it for only the 4th element of the array for example? So I don't want the get the entire array out, just the 4th element.

Answer 1

Use $slice.

db.foo.find({ bar : "xyz" } , { my_array : { $slice : [n , 1] } } )

will retrieve the nth element of the array "my_array" of all documents in the foo collection where bar = "xyz".

Some other examples from the MongoDB documentation:

db.posts.find({}, {comments:{$slice: 5}}) // first 5 comments
db.posts.find({}, {comments:{$slice: -5}}) // last 5 comments
db.posts.find({}, {comments:{$slice: [20, 10]}}) // skip 20, limit 10
db.posts.find({}, {comments:{$slice: [-20, 10]}}) // 20 from end, limit 10

Which you can read here: http://www.mongodb.org/display/DOCS/Retrieving+a+Subset+of+Fields

Answer 2

You can use the $arrayElemAt operator new in MongoDB 3.2 to return the element at the specified array index.

---

Demo:

A collection named baskets contains documents that look like this:

{
    "_id" : ObjectId("578f326f6db61a299a383c5a"),
    "fruits" : [
        "apple",
        "mango",
        "banana",
        "apricot",
        "cherry"
    ]
}

The following query return the element at index -2 (second element) in the "fruits" array.

db.baskets.aggregate(
    [
        { "$project": { "matched": { "$arrayElemAt": [ "$fruits", 1 ] } } } 
    ] 
)

which produces

{ 
    "_id" : ObjectId("578f326f6db61a299a383c5a"), 
    "matched" : "mango" 
}

And the following query the element before the last element in the array; thus the element at index -2

db.baskets.aggregate(
    [
        { "$project": { "matched": { "$arrayElemAt": [ "$fruits", -2 ] } } } 
    ] 
)

which yields:

{ 
    "_id" : ObjectId("578f326f6db61a299a383c5a"), 
    "matched" : "apricot" 
}