如何更新MongoDB文档中的数组元素

Question

I've got a problem updating an array element in MongoDB. This is the structure of a document:

{
        "_id" : ObjectId("57e2645e11c979157400046e"),
        "site" : "BLABLA",
        "timestamp_hour" : 1473343200,
        "values" : [
                {
                        "1473343200" : 66
                },
                {
                        "1473344100" : 230
                },
                {
                        "1473345000" : 479
                },
                {
                        "1473345900" : 139
                }
        ]
}

Now I want to update the element with key "1473345900". How can I do this? I've tried:

db.COLLECTIONNAME.update({"values.1473345900": {$exists:true}}, {$set: {"values.$": 0}})

But after that the document looks like:

{
        "_id" : ObjectId("57e2645e11c979157400046e"),
        "site" : "BLABLA",
        "timestamp_hour" : 1473343200,
        "values" : [
                {
                        "1473343200" : 66
                },
                {
                        "1473344100" : 230
                },
                {
                        "1473345000" : 479
                },
                0
        ]
}

What I'm doing wrong? I only want to update the value of 1473345900 to any value... I don't want to update the complete element...

Thanks a lot!!!

Answer 1

You need to add an additional query in your update that matches the array element you want to update. A typical query would involve checking for the element's value not equal to the one being updated.

The following example update shows this where the $ positional operator identifies the correct index position of the hash key array element { "1473345900": 139 }. If you try to run the update operation without the $ positional operator:

db.COLLECTIONNAME.update(
    { "values.1473345900": { "$exists": true } }, 
    { "$set": { "values.1473345900": 0 } }
)

mongo will treat the timestamp 1473345900 as the index position and thus you will get the error

can't backfill array to larger than 1500000 elements

Thus the correct way should be:

var val = 32;   
db.COLLECTIONNAME.update(
    { "values.1473345900": { "$ne": val, "$exists": true } },
    { "$set": { "values.$.1473345900": val } }
)