How to view document fields in mongo shell? - mongodb

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)

Answer 5

You can do this in a way that gets all the fields even if not every document in the collection has some of them, and without creating a collection:
return db.collectionName.aggregate( [
{ $project : { x : { $objectToArray : "$$ROOT" } } },
{ $unwind : "$x" },
{ $group : { _id : null, keys : { $addToSet : "$x.k" } } },
] ).toArray()[0].keys.sort();
This is also a handy thing to add to the Mongo shell, which you can do by including it your .mongorc.js file in your home directory:
Object.assign( DBCollection.prototype, {
getAllFieldNames() {
return db[ this._shortName ].aggregate( [
{ $project : { x : { $objectToArray : "$$ROOT" } } },
{ $unwind : "$x" },
{ $group : { _id : null, keys : { $addToSet : "$x.k" } } },
] ).toArray()[0].keys.sort();
},
} );
Then you can just do db.myCollection.getAllFieldNames() when using the shell..

Answer 6

var task = db.task.find().next()
for (let key in task){print(key)}