Mongodb upsert embedded document - mongodb

Question

I have a document per day per meter. How can I add another subdocument in the data array and create the whole document if he doesn't exists ?
{
"key": "20120418_123456789",
"data":[
{
"Meter": 123456789,
"Dt": ISODate("2011-12-29T16:00:00.0Z"),
"Energy": 25,
"PMin": 11,
"PMax": 16
}
],
"config": {"someparam": 4.5}
}
Can I use upsert for that purpose ?
The result will be if document exists :
{
"key": "20120418_123456789",
"data":[
{
"Meter": 123456789,
"Dt": ISODate("2011-12-29T16:00:00.0Z"),
"Energy": 25,
"PMin": 11,
"PMax": 16
},
{
"Meter": 123456789,
"Dt": ISODate("2011-12-29T16:15:00.0Z"),
"Energy": 22,
"PMin": 13,
"PMax": 17
}
],
"config": {"someparam": 4.5}
}
Thanks in advance

Answer 1

I think what you want is the $addToSet command - that will push an element to an array only if it does not already exist. I've simplified your example a bit for brevity:
db.meters.findOne()
{
"_id" : ObjectId("4f8e95a718bc9c7da1e6511a"),
"config" : {
"someparam" : 4.5
},
"data" : [
{
"Meter" : 123456789,
}
],
"key" : "20120418_123456789"
}
Now run:
db.meters.update({"key" : "20120418_123456789"}, {"$addToSet": {"data" : {"Meter" : 1234}}})
And we get the updated version:
db.meters.findOne()
{
"_id" : ObjectId("4f8e95a718bc9c7da1e6511a"),
"config" : {
"someparam" : 4.5
},
"data" : [
{
"Meter" : 123456789,
},
{
"Meter" : 1234
}
],
"key" : "20120418_123456789"
}
Run the same command again and the result is unchanged.
Note: you are likely going to be growing these documents, especially if this field is unbounded and causing frequent (relatively expensive) moves by updating in this way - you should have a look here for ideas on how to mitigate this:
http://www.mongodb.org/display/DOCS/Padding+Factor#PaddingFactor-ManualPadding