Get names of all keys in the collection

You could do this with MapReduce:

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

Then run distinct on the resulting collection so as to find all the keys:

db[mr.result].distinct("_id")["foo", "bar", "baz", "_id", ...]

With Kristina's answer as inspiration, I created an open source tool called Variety which does exactly this: https://github.com/variety/variety

You can use aggregation with the new $objectToArray aggregation operator in version 3.4.4 to convert all top key-value pairs into document arrays, followed by $unwind and $group with $addToSet to get distinct keys across the entire collection. (Use $$ROOT for referencing the top level document.)

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

You can use the following query for getting keys in a single document.

db.things.aggregate([  {"$match":{_id: "<<ID>>"}}, /* Replace with the document's ID */  {"$project":{"arrayofkeyvalue":{"$objectToArray":"$$ROOT"}}},  {"$project":{"keys":"$arrayofkeyvalue.k"}}])