When running modifier operations on an embedded object you would often run into situations where in the field name needs to be generated/referenced dynamically when the query runs.

Lets take an example, assume we have a db testing and in that we have a collection testing, in that lets insert some documents like

db.testing.save({key:1,data:{0:1,1:2,2:3}})

using the following commands

> use testing
switched to db testing
> db.testing.remove({})
> db.testing.save({key:1,data:{0:1,1:2,2:3}})
> db.testing.save({key:2,data:{0:1,1:2,2:3}})
> db.testing.save({key:3,data:{0:1,1:2,2:3}})
> db.testing.find()
{ "_id" : ObjectId("4d0259f72715000000004d70"), "key" : 1, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
{ "_id" : ObjectId("4d0259f72715000000004d71"), "key" : 2, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
{ "_id" : ObjectId("4d0259f72715000000004d72"), "key" : 3, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
>

Now if we would want to increment every value of the embedded data(testing.data), we would normally be running a command like

> db.testing.update({key:1},{$inc:{"data.0":1},$inc:{"data.1":2},$inc:{"data.2":3}},false)
> db.testing.find()
{ "_id" : ObjectId("4d0259f72715000000004d70"), "key" : 1, "data" : { "0" : 1, "1" : 2, "2" : 6 } }
{ "_id" : ObjectId("4d0259f72715000000004d71"), "key" : 2, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
{ "_id" : ObjectId("4d0259f72715000000004d72"), "key" : 3, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
>

Now if you noticed we had to build individual increment commands for each field, this may not work out

• when there are large such fields to update, or
• when the field names to update are known at runtime for example in a map-reduce

You cannot use a variable substituion there since the field name in the increment operation maps to a string, if you subtitute it with a variable it would be inserted with a fieldname as the variable.

> var aField = "data.0"
> db.testing.update({key:1},{$inc:{aField:1}},false)
> db.testing.find()
{ "_id" : ObjectId("4d0259f72715000000004d70"), "aField" : 1, "data" : { "0" : 1, "1" : 2, "2" : 6 }, "key" : 1 }
{ "_id" : ObjectId("4d0259f72715000000004d71"), "key" : 2, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
{ "_id" : ObjectId("4d0259f72715000000004d72"), "key" : 3, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
>

Here a new field with the name aField got inserted into the first document

So how do we handle this ? The solution is simple; since the update query takes in a json object, all we need to do is to build the whole thing separately as a json object.
First in-order to bring in the dynamic update use-case, lets use an array that contains the field name as the key and value as the value to be incremented

> var aIncr = [];
> aIncr[0] = 1;
1
> aIncr[1] = 2;
2
> aIncr[2] = 3;
3
> for(pKey in aIncr) {print(pKey);print(aIncr[pKey]);}
0
1
1
2
2
3

This can be built dynamically at runtime. Now we use this to build the update query json object

> for (pKey in aIncr) {
...     incKey = "data." + pKey
...     incData = {}
...     incData[incKey] = aIncr[pKey]
...     incQuery = []
...     incQuery['$inc'] = incData
...     db.testing.update({key:1},incQuery,false)
... }
> db.testing.find()
{ "_id" : ObjectId("4d0259f72715000000004d70"), "aField" : 1, "data" : { "0" : 2, "1" : 4, "2" : 9 }, "key" : 1 }
{ "_id" : ObjectId("4d0259f72715000000004d71"), "key" : 2, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
{ "_id" : ObjectId("4d0259f72715000000004d72"), "key" : 3, "data" : { "0" : 1, "1" : 2, "2" : 3 } }
>

Here we build the increment part as separate json object that contains an array of key-value pairs(the actual fields to increment). Note you would not be able to use a dynamic object instead of an array for the increment json object due to the fact that increment modifier takes an object with a single field and value.

There you go, you can use the same approach when building any such dynamic query.

The complete working script is given below

use testing
db.testing.remove({})
db.testing.save({key:1,data:{0:1,1:2,2:3}})
db.testing.save({key:2,data:{0:1,1:2,2:3}})
db.testing.save({key:3,data:{0:1,1:2,2:3}})
db.testing.find()

var aIncr = [];
aIncr[0] = 1;
aIncr[1] = 2;
aIncr[2] = 3;
for(pKey in aIncr) {print(pKey);print(aIncr[pKey]);}

for (pKey in aIncr) {
	incKey = "data." + pKey
	incData = {}
	incData[incKey] = aIncr[pKey]
	incQuery = []
	incQuery['$inc'] = incData
	db.testing.update({key:1},incQuery,false)
}
db.testing.find()