How to get the last element in a sub document in mongodb - mongodb

Question

This is a sample mongo document i have.
{
"_id" : ObjectId("someid"),
"doc": {
subDoc: [{
content: [{name: 'aaa'}, {name: 'bbb'}, {name: 'ccc'}
},
{
content: [{name: 'aaa'}, {name: 'bbb'}, {name: 'ccc'}
},
{
content: [{name: 'aaa'}, {name: 'bbb'}, {name: 'ccc'}
}]
}
}
I need to get the last element of the content which is nested array as mentioned in the above sample. I tried using aggregate with $project, still i was not able to get the result.
Expected result:
{
"_id" : ObjectId("someid"),
"doc": {
subDoc: [{
content: {name: 'ccc'}
},
{
content: {name: 'ccc'}
},
{
content: {name: 'ccc'}
}]
}
}

Answer 1

Please try this :
db.yourCollectionName.aggregate([{
"$project": {
"doc.subDoc": {
$map:
{
input: "$doc.subDoc",
as: "each",
in: { content: { "$arrayElemAt": ["$$each.content", -1] } }
}
}
}
}])
Collection Data :
/* 1 */
{
"_id" : ObjectId("5e0e6d1f400289966ece90d8"),
"doc" : {
"subDoc" : [
{
"content" : [
{
"name" : "aaa"
},
{
"name" : "bbb"
},
{
"name" : "ccc"
}
]
},
{
"content" : [
{
"name" : "aaa"
},
{
"name" : "bbb"
},
{
"name" : "ccc"
}
]
},
{
"content" : [
{
"name" : "aaa"
},
{
"name" : "bbb"
},
{
"name" : "ccc"
}
]
}
]
}
}
Result :
/* 1 */
{
"_id" : ObjectId("5e0e6d1f400289966ece90d8"),
"doc" : {
"subDoc" : [
{
"content" : {
"name" : "ccc"
}
},
{
"content" : {
"name" : "ccc"
}
},
{
"content" : {
"name" : "ccc"
}
}
]
}
}

Answer 2

Use $slice operator to get the last item from an array in the collection.
{
"$project" : {
"_id" : 1,
"subDoc": { "$slice": [ "$doc.subDoc", -1 ] }
}
}