How can I get and return the first element in an array using a Mongo aggregation?
I tried using this code:
db.my_collection.aggregate([
{ $project: {
resp : { my_field: { $slice: 1 } }
}}
])
but I get the following error:
uncaught exception: aggregate failed: {
"errmsg" : "exception: invalid operator '$slice'",
"code" : 15999,
"ok" : 0
}
Note that 'my_field' is an array of 4 elements, and I only need to return the first element.
Since 3.2, we can use $arrayElemAt to get the first element in an array
db.my_collection.aggregate([
{ $project: {
resp : { $arrayElemAt: ['$my_field',0] }
}}
])
Currently, the $slice operator is unavailable in the the $project operation, of the aggregation pipeline.
So what you could do is,
First $unwind, the my_field array, and then group them together and take the $first element of the group.
db.my_collection.aggregate([
{$unwind:"$my_field"},
{$group:{"_id":"$_id","resp":{$first:"$my_field"}}},
{$project:{"_id":0,"resp":1}}
])
Or using the find() command, where you could make use of the $slice operator in the projection part.
db.my_collection.find({},{"my_field":{$slice:1}})
Update: based on your comments, Say you want only the second item in an array, for the record with an id, id.
var field = 2;
var id = ObjectId("...");
Then, the below aggregation command gives you the 2nd item in the my_field array of the record with the _id, id.
db.my_collection.aggregate([
{$match:{"_id":id}},
{$unwind:"$my_field"},
{$skip:field-1},
{$limit:1}
])
The above logic cannot be applied for more a record, since it would involve a $group, operator after $unwind. The $group operator produces a single record for all the records in that particular group making the $limit or $skip operators applied in the later stages to be ineffective.
A small variation on the find() query above would yield you the expected result as well.
db.my_collection.find({},{"my_field":{$slice:[field-1,1]}})
Apart from these, there is always a way to do it in the client side, though a bit costly if the number of records is very large:
var field = 2;
db.my_collection.find().map(function(doc){
return doc.my_field[field-1];
})
Choosing from the above options depends upon your data size and app design.
$my_field I'll return. I try add $skip, but it isn't a $group operator.
Starting Mongo 4.4, the aggregation operator $first can be used to access the first element of an array:
// { "my_field": ["A", "B", "C"] }
// { "my_field": ["D"] }
db.my_collection.aggregate([
{ $project: { resp: { $first: "$my_field" } } }
])
// { "resp" : "A" }
// { "resp" : "D" }
The $slice operator is scheduled to be made available in the $project operation in Mongo 3.1.4, according to this ticket: https://jira.mongodb.org/browse/SERVER-6074
This will make the problem go away.
This version is currently only a developer release and is not yet stable (as of July 2015). Expect this around October/November time.
Mongo 3.1.6 onwards,
db.my_collection.aggregate([
{
"$project": {
"newArray" : { "$slice" : [ "$oldarray" , 0, 1 ] }
}
}
])
where 0 is the start index and 1 is the number of elements to slice
