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Aggregate a $slice to get an element in exact position from a nested array in MongoDB?
You can use aggregate framework for this. Let us first create a collection with documents −
>db.exactPositionDemo.insertOne({"StudentName":"John","StudentScores":[78,98,56,45,89]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd29a1c345990cee87fd883")
}Following is the query to display all documents from a collection with the help of find() method −
> db.exactPositionDemo.find().pretty();
This will produce the following output −
{
"_id" : ObjectId("5cd29a1c345990cee87fd883"),
"StudentName" : "John",
"StudentScores" : [
78,
98,
56,
45,
89
]
}Case 1 − Query to aggregate a $slice to get an element in exact position with 0,1 −
> db.exactPositionDemo.aggregate([ { "$project": { "StudentScores": { "$slice": ["$StudentScores",0,1] } }} ]);This will produce the following output −
{ "_id" : ObjectId("5cd29a1c345990cee87fd883"), "StudentScores" : [ 78 ] }Case 2 − Query to aggregate a $slice to get an element in exact position with 1,1 −
> db.exactPositionDemo.aggregate([ { "$project": { "StudentScores": { "$slice": ["$StudentScores",1,1] } }} ]);This will produce the following output −
{ "_id" : ObjectId("5cd29a1c345990cee87fd883"), "StudentScores" : [ 98 ] }
Updated on: 30-Jul-2019
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