Get all the names in a MongoDB collection

Originally published on January 18, 2019 at ObjectRocket.com/blog.

To validate your schema, debug for typos in fields, or find fields that you shouldn’t set, you need an understanding of all the keys in your MongoDB® collection.

Many MongoDB-as-a-Service companies, including ObjectRocket, offer an easy way to do this right in the user interface (UI). Seasoned MongoDB users typically start with an object-document mapper (ODM), such as Mongoose for JavaScript® or Mongoengine for Python®, so they can build a consistent schema for their application and reduce typos. ODMs also do type validation, so you don’t accidentally put a string in a field that has integers and has math applied to it.

If you don’t have a service or ODM, you can use several other methods to access the keys. We’ve found the following methods to get all the keys in a MongoDB collection for different circumstances.

Find the MongoDB keys

Suppose you have the following example:

db.activities.insert( { type : [‘indoor’, 'outdoor' , ‘mixed’] } );
db.activities.insert( { activity : 'cycling' } );
db.activities.insert( { activity : ‘skiing’, location: ‘Alpes’, } );
db.activities.insert( { equipment : [‘paddles’,‘sunglasses’] } );

You need to get the unique keys: type, activity, equipment, and location.

MapReduce

You can get the keys with MapReduce by using the following operation:

mr = db.runCommand({ "mapreduce" : "activities", "map" : function() { for (var key in this) { emit(key, null); } }, "reduce" : function(key, stuff) { return null; }, "out": "activities" + "_keys" })

After the operation finishes, run the following distinct command on the resulting collection to find all keys:

 db.activities_keys.distinct("_id")

If you want to get a list of all the unique keys in a subdocument, modify the following line:

for (var key in this.first_level.second_level.nth_level) { emit(key, null); }

Aggregation

You can also use aggregation. This method works with all drivers that support the aggregate framework. With your aggregations, use $sample or $limit to reduce overhead.

Note: Running with primary read preference might impact performance. Consider running with secondary read preference.

Use aggregation with $objectToArrray (available in version 3.4.4 and up) to convert all top key and value pairs into document arrays followed by $unwind and $group with $addToSet to get distinct keys across entire collection.

In the following example, $$ROOT references the top level document.

db.activities.aggregate([ {"$project":{"arrayofkeyvalue":{"$objectToArray":"$$ROOT"}}}, {"$unwind":"$arrayofkeyvalue"}, {"$group":{"_id":null,"allkeys":{"$addToSet":"$arrayofkeyvalue.k"}}} ])

You can also use aggregation to get keys in a single document by using the following command:

db.activities.aggregate([ {"$project":{"arrayofkeyvalue":{"$objectToArray":"$$ROOT"}}}, {"$project":{"keys":"$arrayofkeyvalue.k"}} ])

Mongo shell

You can also get keys under the Mongo shell client by using the following command:

var allKeys = {}; db.YOURCOLLECTION.find().forEach(function(doc){Object.keys(doc).forEach(function(key){allKeys[key]=1})}); allKeys;

Output fields for a single document

Because each document in a collection might have completely different keys, you might need the output of fields for a single document in a collection. Use the following command to do this:

doc=db.thinks.findOne(); for (key in doc) print(key);

Python

You can use Python to get MongoDB keys. The following example returns the set of all top-level keys in the collection:

#Using pymongo and connection named 'db' reduce( lambda all_keys, rec_keys: all_keys | set(rec_keys), map(lambda d: d.keys(), db.things.find()), set() )

JavaScript

JavaScript offers another way to get the keys, as shown in the following example:

db.collection('collectionName').mapReduce( function() { for (var key in this) { emit(key, null); } }, function(key, stuff) { return null; }, { "out": "allFieldNames" }, function(err, results) { var fields = db.collection('allFieldNames').distinct('_id'); fields .then(function(data) { var finalData = { "status": "success", "fields": data }; res.send(finalData); delteCollection(db, 'allFieldNames'); }) .catch(function(err) { res.send(err); delteCollection(db, 'allFieldNames'); }); });

After reading the newly created collection allFieldNames, delete it.

db.collection("allFieldNames").remove({}, function (err,result) { db.close(); return; });

So many options

You’ve got a lot of options to get these Mongo keys. If you’re overwhelmed by MongoDB management and want to focus on your code instead of database management, we’re here for you.

Learn more about Rackspace DBA Services.

Use the Feedback tab to make any comments or ask questions. You can also start a conversation with us.