Consider following collection in mongoDB :
{a:[4,2,8,71,21]}
{a:[24,2,2,1]}
{a:[4,1]}
{a:[4,2,8,21]}
{a:[2,8,71,21]}
{a:[4,2,8]}
How can I get following results in a most easily:
Getting nth element of array
{a:4}
{a:24}
{a:4}
{a:4}
{a:2}
{a:4}
Getting elements 2 to 4
{a:[8,71,21]}
{a:[2,1]}
{a:[]}
{a:[8,21]}
{a:[71,21]}
{a:[8]}
And other similar queries.
What you are looking for is the $slice projection.
You can pass a simple $limit with a number of values to return (eg. 1):
> db.mycoll.find({}, {_id: 0, a: { $slice: 1}})
{ "a" : [ 4 ] }
{ "a" : [ 24 ] }
{ "a" : [ 4 ] }
{ "a" : [ 4 ] }
{ "a" : [ 2 ] }
{ "a" : [ 4 ] }
You can pass an array with parameters of ( $skip, $limit ).
Note: to match your expected output you would have to find elements 3 to 5 (skip the first 2 elements, return the next 3):
> db.mycoll.find({}, {_id: 0, a: { $slice: [2,3]}})
{ "a" : [ 8, 71, 21 ] }
{ "a" : [ 2, 1 ] }
{ "a" : [ ] }
{ "a" : [ 8, 21 ] }
{ "a" : [ 71, 21 ] }
{ "a" : [ 8 ] }
Pass the number of elements to $skip and a value of 1 for the limit.
For example, to find the second element you need to skip 1 entry:
> db.mycoll.find({}, {_id: 0, a: { $slice: [1,1]}})
{ "a" : [ 2 ] }
{ "a" : [ 2 ] }
{ "a" : [ 1 ] }
{ "a" : [ 2 ] }
{ "a" : [ 8 ] }
{ "a" : [ 2 ] }
$slice operator:
$slice operator always returns an array. I don't believe there is syntax (as at MongoDB 2.2) to project a single simple array element as a scalar. You could achieve this with MapReduce, but that seems like overkill compared to just converting the single element array returned by $slice to a scalar in your application code if needed. If your array contained embedded documents and you wanted to match specific values (not an array position) you could use the $elemMatch projection to return the first matching element.