need get all key from object in mongodb
$objectToArrayconvertspecobject to array in key-value format$unwinddeconstructspecarray$groupby null and construct the unique array ofspecobject's key using$addToSet
db.collection.aggregate([ { $project: { spec: { $objectToArray: "$spec" } } }, { $unwind: "$spec" }, { $group: { _id: null, spec: { $addToSet: "$spec.k" } } }])