How can I ask MongoDB to evaluate some JavaScript in order to obtain value for a field?

Question

I want to let MongoDB dynamically assign a value to one of the fields of the document I'm inserting. For example: the current time from MongoDB server just like NOW() would do in MySQL.

I tried this:

c := mongoSession.DB("myapp").C("instances")
rand.Seed(time.Now().UnixNano())
err := c.Insert(
   struct{Serial, Priority, Url, LastSeen interface{}}{ 
      Url: getInformedHost() + ":" + getRunningPortString(), 
      Priority: rand.Int(), 
      LastSeen: mongoNow() }
)
checkError(err, "Could not register on MongoDB server.", 3)

I have this helper function:

func mongoNow() bson.JavaScript {
    return bson.JavaScript{Code: 
         "(new Date()).ISODate('YYYY-MM-DD hh:mm:ss')"}
}

the LastSeen field gets stored as a script instead of evaluated:

[_id] => MongoId Object (
    [$id] => 502d6f984eaead30a134fa10
)
[priority] => 1694546828
[url] => 127.0.0.1:8080
[lastseen] => MongoCode Object (
    [code] => (new Date()).ISODate('YYYY-MM-DD hh:mm:ss')
    [scope] => Array (
    )
)

How can I get some javascript evaluated instead of inserted?

Answer 1

See the following URL on the MongoDB documentation:

http://www.mongodb.org/display/DOCS/Server-side+Code+Execution#Server-sideCodeExecution-Storingfunctionsserverside

There is a special system collection called system.js that can store JavaScript functions to be reused.

Note though, that the support and performance of server-sided code (equivalent to stored procedures) is still a little poor (details in link).

Edit:

To call a stored procedure from Go using the mgo driver use the mgo.Database type's Run() method (direct link) and issue an eval command with the Javascript code to be executed server-side as argument. Something like:

db.Run(bson.M{"eval": "myStoredFunction();"})

_{code untested}

It is not possible to have code evaluated in a MongoDB insert statement.