MongoDB: _id Cannot Be An Array - mongodb

Question

I have a large dataset (about 1.1M documents) that I need to run mapreduce on.
The field to group on is an array named xref. Due to the size of the collection and the fact I'm doing this in a 32-bit environment, I'm trying to reduce the collection to another collection in a new database.
First, here's a data sample:
{ "_id" : ObjectId("4ec6d3aa61910ad451f12e01"),
"bii" : -32.9867,
"class" : 2456,
"decdeg" : -82.4856,
"lii" : 297.4896,
"name" : "HD 22237",
"radeg" : 50.3284,
"vmag" : 8,
"xref" : ["HD 22237", "CPD -82 65", "-82 64","PPM 376283", "SAO 258336",
"CP-82 65","GC 4125" ] }
{ "_id" : ObjectId("4ec6d44661910ad451f78eba"),
"bii" : -32.9901,
"class" : 2450,
"decdeg" : -82.4781,
"decpm" : 0.013,
"lii" : 297.4807,
"name" : "PPM 376283",
"radeg" : 50.3543,
"rapm" : 0.0357,
"vmag" : 8.4,
"xref" : ["HD 22237", "CPD -82 65", "-82 64","PPM 376283", "SAO 258336",
"CP-82 65","GC 4125" ] }
{ "_id" : ObjectId("4ec6d48a61910ad451feae04"),
"bii" : -32.9903,
"class" : 2450,
"decdeg" : -82.4779,
"decpm" : 0.027,
"hd_component" : 0,
"lii" : 297.4806,
"name" : "SAO 258336",
"radeg" : 50.3543,
"rapm" : 0.0355,
"vmag" : 8,
"xref" : ["HD 22237", "CPD -82 65", "-82 64","PPM 376283", "SAO 258336",
"CP-82 65","GC 4125" ] }
Here are the map and reduce functions (right now I'm only lii and bii fields):
function map() {
try {
emit(this.xref, {lii:this.lii, bii:this.bii});
} catch(e) {
}
}
function reduce(key, values) {
var result = {xref:key, lii: 0.0, bii: 0.0};
try {
values.forEach(function(value) {
if (value.lii && value.bii) {
result.lii += value.lii;
result.bii += value.bii;
}
});
result.bii /= values.length;
result.lii /= values.length;
} catch(e) {
}
return result;
}
Unfortunately, running this eventually comes up with an error message:
db.catalog.mapReduce(map, reduce, {out:{replace:"catalog2", db:"astro2"}});
Wed Nov 23 10:12:25 uncaught exception: map reduce failed:{
"assertion" : "_id cannot be an array",
"assertionCode" : 10099,
"errmsg" : "db assertion failure",
"ok" : 0
The xref field IS an array, but all values are equal in that array. Is it trying to use that array as the id field in the new collections?

Answer 1

Yes it is not possible to set _id as an array, because it has a special behavior for indexing.
The key you emit by is used as _id in the output collection.
Potentially this could work only with an "inline" output mode if the result is small, since it wont go to a collection.
But ideally you would translate the array into a string (for example concat the values) and use that as _id, or make it a sub-object instead of an array.
Also note that the result of your reduce function should not include the key.
Just return {lii: .., bii: ..}