$size operator is inserted as static string in mongodb upsert - mongodb

Question

In MongoDB, I want to have a document similar to this:
{
"project": "myproject",
"tasks": ["mytask"],
"tasksCount": 1
}
where tasksCount is the length of the tasks array. I use this upsert query as suggested here:
db.tasks.updateOne(
{ project: "myproject" },
{
$addToSet: { tasks: "mytask" },
$set: { tasksCount: { $size: "$tasks" } }
},
{ upsert: true }
)
But the actual document inserted, looks like this:
{
"project": "myproject",
"tasks": ["mytask"],
"tasksCount": {
"$size": "$tasks"
}
}
where the tasksCount is simply the object I pass in, instead of the actual size.
I am using mongo v5.0.8.

Answer 1

As #turvishal mentioned in the comment $size is an aggregation operator , here is an option using update with aggregation framework:
db.collection.update({
project: "myproject"
},
[
{
$project: {
tasks: {
$setUnion: [
"$tasks",
[
"mytask2"
]
]
}
}
},
{
$addFields: {
tasksCount: {
$size: "$tasks"
}
}
}
],
{
upsert: true,
multi: true
})
Explained:
Match all documents with project:"myproject"
Project $setUnion ( addToSet) to add the task to the tasks array.
Replace the field tasksCount with the size of the tasks array.
Upsert:true + multi:true to perform the operation for all matching and insert if there is no matchiing.
Plaground