Stored JS in MongoDB is saved in the special system.js collection. The documents there should be structured like:

{_id: "sum",
 value: function (x, y) { return x + y; }}

_id is the name of the function, and value is the JS code defining the function. Here’s an example of saving such a function and then using it in an eval. This can be run using the MongoDB shell:

> db.system.js.save({_id: "sum",
...                  value: function (x, y) { return x + y; }});
> db.eval("return sum(2, 3);");
5

Once sum is defined we can use it from all of MongoDB’s JS contexts, including Map/Reduce and $where. In this example we use sum within a $where clause to get all documents where the sum of x and y is 6:

> db.test.save({x: 4, y: 2});
> db.test.save({x: 4, y: 3});
> db.test.save({x: 3, y: 3});
> db.test.find({$where: "sum(this.x, this.y) == 6"});
{ "_id" : ObjectId("4bba376231d25858659843f7"), "x" : 4, "y" : 2 }
{ "_id" : ObjectId("4bba376e31d25858659843f9"), "x" : 3, "y" : 3 }

Since system.js is a collection, we can perform normal MongoDB operations on it in order to administer our stored JS functions. We might want to list the stored functions:

> db.system.js.distinct("_id");
[ "sum" ]

or remove a function we previously stored:

> db.system.js.remove({_id: "sum"});
> db.eval("return sum(2, 3);");
Mon Apr  5 15:14:45 JS Error: uncaught exception: {
    "errno" : -3,
    "errmsg" : "[...] sum is not defined [...]",
    "ok" : 0
}

Stored JS and PyMongo

In version 1.5 PyMongo added some helpers to make working with stored JS incredibly easy. The API is accessed through the Database.system\_js property, which returns an instance of the SystemJS helper class. The SystemJS class allows us to add, call, and remove stored JS functions using a nice Pythonic interface:

>>> db.system_js.sum = "function (x, y) { return x + y; }"
>>> db.system_js.sum(3, 2)
5.0
>>> del db.system_js.sum
>>> db.system_js.sum(3, 2)
Traceback (most recent call last):
pymongo.errors.OperationFailure: [...] sum is not defined [...]

The implementation of the SystemJS class is actually pretty trivial. The trickiest bit is getting the argument passing right between the Python lambda returned by *getattr* and the invocation of the stored JS function within the eval.