> db.b.findOne()
{
"_id" : ObjectId("4d9785a5e8cf7bf87ef7a681"),
"groups" : [

{ "groupId" : ObjectId("4d9733172eda11cf36199d6b"), "addTime" : ISODate("2011-04-02T20:28:46.442Z") }

,

{ "groupId" : ObjectId("4d9733172eda11cf36199d6c"), "addTime" : ISODate("2011-04-02T20:28:46.442Z") }

]
}
// I made a mistake here when it should have included "groups" like this: { $pull : { groups : { groupId....
> db.b.update({_id:ObjectId("4d9785a5e8cf7bf87ef7a681")}, {$pull: { groupId:new ObjectId("4d9733172eda11cf36199d6b")}})
> db.getLastErrorObj()
{
"updatedExisting" : true,
"n" : 1,
"connectionId" : 16,
"err" : null,
"ok" : 1
}
//doesn't look updated to me.
> db.b.findOne()
{
"_id" : ObjectId("4d9785a5e8cf7bf87ef7a681"),
"groups" : [

{ "groupId" : ObjectId("4d9733172eda11cf36199d6b"), "addTime" : ISODate("2011-04-02T20:28:46.442Z") }

,

{ "groupId" : ObjectId("4d9733172eda11cf36199d6c"), "addTime" : ISODate("2011-04-02T20:28:46.442Z") }

]
}