Upserting a value at an array position using $min - arrays

Question

I would like to either insert a new document with a default value as part of an array, or update that part of the array if the document already exists.
What I thought of was:
db.test.update(
{ "a": 5 },
{ $setOnInsert: { "b": [0, 0] }, $min: { "b.0": 5 } },
{ upsert: true }
);
If I do that, then I get:
Cannot update 'b' and 'b.0' at the same time
Another idea was to remove $setOnInsert and just keep $min, since the minimum between nothing and 5 should be 5.
db.test.update(
{ "a": 5 },
{ $min: { "b.0": 5 } },
{ upsert: true }
);
This doesn't raise an error, but now the document I get is:
{ "a" : 5, "b" : { "0" : 5 } }
I need an array with 5 at position 0 however, not an object with a 0 property.
How can I achieve this?

Answer 1

You can use .bulkWrite() for this, and it's actually a prime use case of why this exists. It only sends "one" actual request to the server and has only one response. It's still two operations, but they are more or less tied together and generally atomic anyway:
db.junk.bulkWrite([
{ "updateOne": {
"filter": { "a": 1 },
"update": { "$setOnInsert": { "b": [ 5,0 ] } },
"upsert": true
}},
{ "updateOne": {
"filter": { "a": 1 },
"update": { "$min": { "b.0": 5 } }
}}
])
Run for the first time will give you an "upsert", note that it's "inserted" and not "modified" in the response:
{
"acknowledged" : true,
"deletedCount" : 0,
"insertedCount" : 0,
"matchedCount" : 1,
"upsertedCount" : 1,
"insertedIds" : {
},
"upsertedIds" : {
"0" : ObjectId("5947c412d6eb0b7d6ac37f09")
}
}
And the document of course looks like:
{
"_id" : ObjectId("5947c412d6eb0b7d6ac37f09"),
"a" : 1,
"b" : [
5,
0
]
}
Then run with a different value to $min as you likely would in real cases:
db.junk.bulkWrite([
{ "updateOne": {
"filter": { "a": 1 },
"update": { "$setOnInsert": { "b": [ 5,0 ] } },
"upsert": true
}},
{ "updateOne": {
"filter": { "a": 1 },
"update": { "$min": { "b.0": 3 } }
}}
])
And the response:
{
"acknowledged" : true,
"deletedCount" : 0,
"insertedCount" : 0,
"matchedCount" : 2,
"upsertedCount" : 0,
"insertedIds" : {
},
"upsertedIds" : {
}
}
Which "matched" 2 but of course $setOnInsert does not apply, so the result is:
{
"_id" : ObjectId("5947c412d6eb0b7d6ac37f09"),
"a" : 1,
"b" : [
3,
0
]
}
Just like it should be