I have many entities with a big array for each on these entities in DB. I want to retrieve the first and the last element of the array (there is too much data if I get all the arrays). How can I do it ? I tried this:
db.my_collection.findOne({my_query:'is_awesome'}, {'big_array.0':1})
but it doesn't work... Thank's !
You can use an aggregation instead of findOne, performing a $match.
In Mongodb 3.2, there is $slice in aggregation, so you can $slice the item at position 1 and -1 :
db.my_collection.aggregate([{
$match: { my_query: 'is_awesome' }
}, {
$project: {
my_query: 1,
firstElement: { $slice: ["$big_array", 1] },
lastElement: { $slice: ["$big_array", -1] }
}
}])
In Mongodb < 3.2, $slice cant be used in aggregation, so you can use an $unwind and $group, the following is working in Mongodb 3.0 :
db.my_collection.aggregate([{
$match: { my_query: 'is_awesome' }
}, {
$unwind: "$big_array"
}, {
$group: {
_id: "$_id",
my_query: { $first: "$my_query" },
firstElement: { $first: "$big_array" },
lastElement: { $last: "$big_array" }
}
}])
mongo --version give me 3.0.8. Then I have the following error with your trick : assert: command failed: { "errmsg" : "exception: invalid operator '$slice'", "code" : 15999, "ok" : 0 } : aggregate failed Error: command failed: { "errmsg" : "exception: invalid operator '$slice'", "code" : 15999, "ok" : 0 } : aggregate failed Did I miss something ?
$slice in aggregation is only for 3.2+, I've updated my post with a solution working in 3.0