MongoDB: updating the value of an inner BSON - mongodb

Question

I have a document with a schema in mongodb that looks like this:
{
"_id" : ObjectId("572f88424de8c74a69d4558c"),
"storecode" : "ABC",
"credit" : true,
"group" : [
{
"group_name" : "Frequent_Buyer",
"time" : NumberLong("1462732865712"),
}
],
}
I want to add on the _id part for the first object in the array under group so it looks like this:
{
"_id" : ObjectId("572f88424de8c74a69d4558c"),
"storecode" : "ABC",
"credit" : true,
"group" : [
{
"group_name" : "Frequent_Buyer",
"time" : NumberLong("1462732865712"),
"_id" : "573216fee4430577cf35e885"
}
],
}
When I try this code it fails:
db.customer.update({ "_id": ObjectId("572f88424de8c74a69d4558c") },{ "$set": { "group.$._id": "573216fee4430577cf35e885"} })
WriteResult({
"nMatched" : 0,
"nUpserted" : 0,
"nModified" : 0,
"writeError" : {
"code" : 16837,
"errmsg" : "The positional operator did not find the match needed from the query. Unexpanded update: groups.$._id"
}
})
However, if I adjust the code slightly and add on an extra criteria for querying, it works:
db.customer.update({ "_id": ObjectId("572f88424de8c74a69d4558c"), "group.groupname": "Frequent_Buyer" },{ "$set": { "group.$._id": "573216fee4430577cf35e885"} })
Results:
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
Why did the first command not work but the second command work?

Answer 1

This is the expected result. To use the positional $ update operator, the array field must appear as part of the query document as mentioned in the documentation.
When used with update operations, e.g. db.collection.update() and db.collection.findAndModify(),
the positional $ operator acts as a placeholder for the first element that matches the query document, and
the array field must appear as part of the query document.
For example If you don't want to filter your documents using group_name, simply add group: { "$exists": true } or "group.0": { "$exists": true } to your query criteria. You query will then look like this:
db.customer.updateOne(
{
"_id": ObjectId("572f88424de8c74a69d4558c"),
"group.0": { "$exists": true}
},
{ "$set": { "group.$._id": "573216fee4430577cf35e885" } }
)
Last and not least, you should be using updateOne or updateMany because update is deprecated in official language driver.