Golang/mgo: How can I ask MongoDB to use current time in a field?

Question

I have this struct that matches the types of a MongoDB collection I'm using:

type AppInstance struct {
    Id bson.ObjectId "_id,omitempty"
    Url string
    Priority int
    LastSeen string
}

I want the LastSeen field to hold the time of the last interaction with that particular app. So, the app registers itself setting current time (as a string).

What I would like is Mongo to dynamically set its own current time into that field when it inserts, just like MySQL's NOW() function would do.

I have this helper function:

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

And I tried this:

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

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

[_id] => MongoId Object (
    [$id] => 502d6f984eaead30a134fa10
)
[id] => MongoId Object (
    [$id] => 502d6f98aa443e0ffd000001
)
[priority] => 1694546828
[url] => 127.0.0.1:8080
[lastseen] => MongoCode Object (
    [code] => (new Date()).ISODate('YYYY-MM-DD hh:mm:ss')
    [scope] => Array (
    )
)

So, I think there are to questions:

First, how can I insert the current time?

Second, how can I get some javascript evaluated instead of inserted?

The answer to the second one could be enough to answer the first one, but it might as well not be.

Answer 1

Don't store time as string. mgo supports time.Time which is like a Date object in Javascript:

type Event struct {
    Id    bson.ObjectId "_id,omitempty"
    Which string
    Date  time.Time
}

Insert an event that happened now:

e := Event{
    Which: "first event",
    Date: time.Now(),
}
c.Insert(e)

Answer 2

In Mongo 2.6 you can do it natively with $currentDate operator.

db.users.update( { _id: 1 }, {
   $currentDate: {
     lastModified: true,
     lastModifiedTS: { $type: "timestamp" }
   },
   $set: { status: "D" }
})