Where does MongoDB store function added via console? - mongodb

Question

I created auto-incrementing sequence field via counters collection and getNextSequence() function (absolutely like in docs)
According another document JavaScript functions are stored in a special system collection named system.js
But there is no such collection at my database (a least db.system.js.find() shows empty result):
> db.dropDatabase();
{ "dropped" : "mongopa", "ok" : 1 }
> version()
3.2.5
> db.counters.insert({_id: "userid", seq: 0 })
WriteResult({ "nInserted" : 1 })
> db.counters.find()
{ "_id" : "userid", "seq" : 0 }
> function getNextSequence(name) {
... var ret = db.counters.findAndModify(
... {
... query: { _id: name },
... update: { $inc: { seq: 1 } },
... new: true
... }
... );
...
... return ret.seq;
... }
> db.system.js.find()
> show collections
counters
> db.users.insert({"login":"demo","user_id":getNextSequence("userid"),"password":"demo"})
WriteResult({ "nInserted" : 1 })
> db.users.find()
{ "_id" : ObjectId("574ff1c7436a1b4f9c6f47b9"), "login" : "demo", "user_id" : 1, "password" : "demo" }
> db.users.insert({"login":"demo2","user_id":getNextSequence("userid"),"password":"demo2"})
WriteResult({ "nInserted" : 1 })
> db.users.find()
{ "_id" : ObjectId("574ff1c7436a1b4f9c6f47b9"), "login" : "demo", "user_id" : 1, "password" : "demo" }
{ "_id" : ObjectId("574ff1d6436a1b4f9c6f47ba"), "login" : "demo2", "user_id" : 2, "password" : "demo2" }
>
So where does the getNextSequence function really stored?

Answer 1

When you define the function as,
function getNextSequence(name) {
var ret = db.counters.findAndModify(
{
query: { _id: name },
update: { $inc: { seq: 1 } },
new: true
}
);
return ret.seq;
}
It is merely defined for that particular session and is not available to you once the session ends. Hence, its not saved anywhere.
To make the function re-usable across the sessions, you need to explicitly save the function is system.js by using,
db.system.js.save(
{
_id: "getNextSequence",
value: function(name){var ret = db.counters.findAndModify({
query: { _id: name },
update: { $inc: { seq: 1 } },
new: true
});
return ret.seq;}
})
Once you have saved the function, you can cross-check it by,
db.system.js.find()
You need to call this
db.loadServerScripts();
across the sessions. It loads all the scripts saved in system.js collection.
For details, please check here.