How to view document fields in mongo shell?

Question

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"

Answer 1

Even easier:

Object.keys(db.messages.findOne())

Answer 2

A for ... in loop should do the trick:

> var message = db.messages.findOne();
> for (var key in message) {
... print(key);
... }

Answer 3

Other answers are correct.

However, as I am completely new, I didn't understand where & how the above commands need to be executed.

Below helped, from my github.
On Windows: Run this code in a command prompt (cmd).
On Mac or Linux: Run this code in a terminal window.

// ------------
// start mongo client
mongo

// ------------

// list all databases
show dbs
// NOTE: assume one of the databases is myNewDatabase

// use the 'myNewDatabase' database
use myNewDatabase

// ------------

// show all collections of 'myNewDatabase' database
show collections
// NOTE: assume one of the collections is 'myCollection'

// show all documents of 'myCollection' collection
db.myCollection.find()

// ------------

// field keys
Object.keys(db.myCollection.findOne());

// values
db.myCollection.find().forEach(function(doc) {
    for (field in doc) {
        print(doc[field]);
    }
});

// ------------

Answer 4

To get a list of all fields used in a collection in MongoDB, this is the way I found most straightforward (your mileage may vary :) ):

Create a .js file with the content:

use yourdbname
mr = db.runCommand({
  "mapreduce" : "collectionName",
  "map" : function() {
    for (var key in this) { emit(key, null); }
  },
  "reduce" : function(key, stuff) { return null; },
  "out": "collectionName" + "_keys"
})
db[mr.result].distinct("_id")

I found out how to do this here (GeoffTech blog)

I ran it from the shell to print the output in the console

mongo < nameOfYourFile.js 

or dump the output in a text file:

mongo < nameOfYourFile.js > outputDir\nameOfYourOutputFile.txt

I'm totally new to MongoDb so I hope it does indeed get all fields regardless of use throughout the documents!

(I'm using MongoDb on windows 10, so my console may differ from yours)