Mongodb how to update last item in array - mongodb

Question

can somebody tell me please if is possible to update last item in array of documents? For example in this document:
{
name: 'my name',
someArray: [
{rate: 10},
{rate: 9},
{rate: 20}
]
}
I would like to update last item with rate:20 to rate: 50.
How to update exactly the last item in Mongodb?
Thanks.

Answer 1

One option is using an update pipeline:
Split the array into the last item and all the rest.
update the last item
Build the array again using $concatArrays
db.collection.update(
{name: "my name"},
[
{$set: {
lastItem: {$last: "$someArray"},
rest: {$slice: ["$someArray", 0, {$subtract: [{$size: "$someArray"}, 1]}]}
}
},
{$set: {"lastItem.rate": 50}},
{$set: {
someArray: {$concatArrays: ["$rest", ["$lastItem"]]},
lastItem: "$$REMOVE",
rest: "$$REMOVE"
}
}
])
See how it works on the playground example - concatArrays
Another option is using $reduce:
Here we are iterating on the array, for each item checking if it is the last one, and if so updating it:
db.collection.update(
{name: "my name"},
[
{$set: {lastIndex: {$subtract: [{$size: "$someArray"}, 1]}}},
{$set: {
lastIndex: "$$REMOVE",
someArray: {
$reduce: {
input: "$someArray",
initialValue: [],
in: {
$concatArrays: [
"$$value",
[
{
$cond: [
{$eq: [{$size: "$$value"}, "$lastIndex"]},
{$mergeObjects: [
"$$this",
{rate: 50}
]
},
"$$this"
]
}
]
]
}
}
}
}
}
])
See how it works on the playground example - reduce