Get all fields names in a MongoDB collection?

To get all field names in a MongoDB collection, you can use Map-Reduce with the distinct() method. This approach extracts all unique field names across all documents in the collection.

Syntax

db.runCommand({
    "mapreduce": "collectionName",
    "map": function() {
        for (var key in this) { emit(key, null); }
    },
    "reduce": function(key, values) { return null; },
    "out": "temp_collection"
});

db.temp_collection.distinct("_id");

Sample Data

Let us create a collection with sample documents ?

db.getAllFieldNamesDemo.insertMany([
    {"StudentFirstName": "David", "StudentAge": 23},
    {"StudentFirstName": "John", "StudentAge": 25, "StudentCity": "New York"},
    {"StudentLastName": "Miller", "StudentAge": 22, "StudentGrade": "A"}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cd998e9b50a6c6dd317ad90"),
        ObjectId("5cd998e9b50a6c6dd317ad91"),
        ObjectId("5cd998e9b50a6c6dd317ad92")
    ]
}

Display Sample Documents

db.getAllFieldNamesDemo.find();

{ "_id": ObjectId("5cd998e9b50a6c6dd317ad90"), "StudentFirstName": "David", "StudentAge": 23 }
{ "_id": ObjectId("5cd998e9b50a6c6dd317ad91"), "StudentFirstName": "John", "StudentAge": 25, "StudentCity": "New York" }
{ "_id": ObjectId("5cd998e9b50a6c6dd317ad92"), "StudentLastName": "Miller", "StudentAge": 22, "StudentGrade": "A" }

Get All Field Names Using Map-Reduce

myMapReduce = db.runCommand({
    "mapreduce": "getAllFieldNamesDemo",
    "map": function() {
        for (var myKey in this) { emit(myKey, null); }
    },
    "reduce": function(myKey, values) { return null; },
    "out": "getAllFieldNamesDemo_keys"
});

db[myMapReduce.result].distinct("_id");

{
    "result": "getAllFieldNamesDemo_keys",
    "timeMillis": 1375,
    "counts": {
        "input": 3,
        "emit": 9,
        "reduce": 0,
        "output": 6
    },
    "ok": 1
}

["StudentAge", "StudentCity", "StudentFirstName", "StudentGrade", "StudentLastName", "_id"]

How It Works

• The map function iterates through each field in every document and emits the field name as a key.
• The reduce function is not needed here, so it returns null.
• The result is stored in a temporary collection specified by the "out" parameter.
• distinct("_id") extracts all unique field names from the temporary collection.

Conclusion

Map-Reduce with distinct() effectively extracts all unique field names from a MongoDB collection. This method works across all documents and captures every field that exists in the collection.