mongoose "_id" vanishing in collection created with map/reduce - node.js

Question

I done a very simple map/reduce in mongo console.
var mapState = function () {
emit(this.state, 1);
};
var sumState = function (keyState, valuesCount) {
return Array.sum(valuesCount);
};
db.FooBar.mapReduce(
mapState,
sumState,
{out: "state_counts"}
);
var sc = {};
db.state_counts.find(
{_id: {$exists: true}}).forEach(
function(o){
sc[o._id]=o.value;
}
);
> sc
{
"ak" : 29,
"al" : 5832,
"ar" : 2798,
...
}
> db.state_counts.find().limit(3)
{ "_id" : "ak", "value" : 29 }
{ "_id" : "al", "value" : 5832 }
{ "_id" : "ar", "value" : 2798 }
So far so good. I have the expected state abbreviations and counts in the "sc" object. Oddness occurs when I'm attempting to pull data from state_counts prior to converting it to the equivalent of the "sc" object using mongoose.
#!/usr/bin/env node
mongoose = require("mongoose");
mongoose.connect("mongodb://localhost/thedb");
var schema = new mongoose.Schema({});
schema.collection = 'state_counts';
console.log(schema.collection);
var cur = mongoose.model(schema.collection, schema);
cur.find({}).exec(
function(err, data) {
if (err) {
console.log(err);
mongoose.disconnect();
}
console.log(data);
mongoose.disconnect();
}
);
$ ./test.js
state_counts
[ { value: 29 },
{value: 5832 },
{ value: 2798 },
...
]
This is surprising to me. Why is the "_id" value not showing up in my script when using mongoose?

Answer 1

_id isn't showing up because you haven't defined a schema and mongoose is all about adding schemas to mongodb. So given a completely empty schema, mongoose probably assumes _id will be of type ObjectId (which is conventional for mongodb) and when casting the data in mongodb to that type fails, as it will always do given your data, mongoose omits the value, which makes sense given the majority of mongoose's job is to enforce a consistent schema. This will "fix" it.
var schema = new mongoose.Schema({_id: String, value: Number});