How to view document fields in mongo shell?

Is there a way to figure out the fields/keys in a document while in mongo's shell? As an example, let's say we have a document like (pseudocode):

{
    "message": "Hello, world",
    "from": "hal",
    "field": 123
}

I'd like to run a command in the shell that returns the list of fields/keys in that document. For instance, something like this:

> var message = db.messages.findOne()
> message.keys()
... prints out "message, from, field"