Update document inside an array using $where and $and - mongodb

Question

I've document like the one below, I want to update sold field only when the sold is less than the total field. I know how to retrieve a document within a array of documents and I know how to compare two fields in a document, but I don't know how can I do them all together in one single expression along with the update.
{
"_id" : ObjectId("4f7ee46e08403d063ab0b4f9"),
"name" : "MongoDB",
"notes" : [
{
"_Id" : ObjectId("4f7ee46e08403d063ab0h659")
"title" : "Hello MongoDB",
"content" : "Hello MongoDB",
"total" : 100
},
{
"_Id" : ObjectId("4f7ee46e08403d083ab0h659")
"title" : "ReplicaSet MongoDB",
"content" : "ReplicaSet MongoDB",
"total" : 200,
"sold" : 2
},
{
"_Id" : ObjectId("4f7ee65e08403d083ab0h659")
"title" : "ReplicaSet MongoDB",
"content" : "ReplicaSet MongoDB",
"total" : 200,
}
]
}
My expression to read the document from the array of documents:
db.Collection.find({'notes._id':ObjectId('58791af46c698c00475e7f41')},{'notes.$':true})
I can write an expression like this to compare two fields
db.Collection.find( { $where : "this.total > this.sold" });

Answer 1

The best approach to this would be to first use aggregation as means to determine a list of documents as well as a subset of the array elements that match the given condition i.e. where sold is less than the total field.
Once you get this list you can then process Bulk updates which will at least allow many operations to be sent in a single request with a singular response.
The following example shows how you can go about this:
var ops = [],
cursor = db.collection.aggregate([
{
"$redact": {
"$cond": [
{
"$allElementsTrue": {
"$map": {
"input": "$notes",
"as": "el",
"in": { "$gt": [ "$$el.total", "$$el.sold" ] }
}
}
},
"$$KEEP",
"$$PRUNE"
]
}
},
{
"$project": {
"notes": {
"$filter": {
"input": "$notes",
"as": "el",
"cond": {
"$and": [
{ "$gt": [ "$$el.total", "$$el.sold" ] },
{ "$gt": [ "$$el.sold", null ] }
]
}
}
}
}
}
]);
cursor.forEach(function(doc) {
doc.notes.forEach(function(note){
ops.push({
"updateOne": {
"filter": { "_id": doc._id, "notes._id": note._id },
"update": {
"$set": { "notes.$.sold": 400 } // update the sold field accordingly
}
}
});
if (ops.length === 500 ) {
db.collection.bulkWrite(ops);
ops = [];
}
});
})
if (ops.length > 0)
db.collection.bulkWrite(ops);