How do I unset all fields except a known set of fields?

Question

Suppose I have a single document in my mongo collection that looks like this:

{
    "_id": 123,
    "field_to_prune": 
    {
        "keep_field_1": "some value",
        "random_field_1": "some value",
        "keep_field_2": "some value",
        "random_field_2": "some value",
        "random_field_3": "some value"
    }
}

I want to prune that document to look like this:

{
    "_id": 123,
    "field_to_prune": 
    {
        "keep_field_1": "some value",
        "keep_field_2": "some value"
    }
}

However, my issue is that I don't know what the "random" field names are. In mongo, how would i $unset all fields except a couple of known fields?

I can think of a couple of ways, but i don't know the syntax.. i could select all field NAMES and then for each one of those unset the field. kind of like this:

[Some query to find all field names under "field_to_prune" for id 123].forEach(function(i) { 
    var key = "field_to_prune." + i;
    print("removing field: " + key);
    var mod = {"$unset": {}};
    mod["$unset"][key] = "";

    db.myCollection.update({ _id: "123" }, mod);
});

Another way I was thinking of doing it was to unset where the field name is not in an array of strings that i defined. not sure how to do that either. Any ideas?

Answer 1

Actually the best way to do this is to iterate over the cursor an use the $unset update operate to remove those fields in subdocuments except the known fields you want to keep. Also you need to use "bulk" operations for maximum efficiency.

---

MongoDB 3.2 deprecates Bulk() and its associated methods. So if you should use the .bulkWrite()

var count = 0;
var wantedField = ["keep_field_1", "keep_field_2"]; 


var requests = [];
var count = 0;
db.myCollection.find().forEach(function(document) { 
    var fieldToPrune = document.field_to_prune; 
    var unsetOp = {};
    for (var key in fieldToPrune) {     
        if ((wantedFields.indexOf(key) === -1) && Object.prototype.hasOwnProperty.call(fieldToPrune, key ) ) {
            unsetOp["field_to_prune."+key] = " ";        
        }
    }
    requests.push({ 
        "updateOne": { 
            "filter": { "_id": document._id }, 
            "update": { "$unset": unsetOp } 
         }
    });         
    count++;    
    if (count % 1000 === 0) {   
        // Execute per 1000 operations and re-init  
        db.myCollection.bulkWrite(requests); 
        requests = []; 
    } 
})

// Clean up queues
db.myCollection.bulkWrite(requests)

---

From MongoDB 2.6 you can use the Bulk API.

var bulk =  db.myCollection.initializeUnorderedBulkOp();
var count = 0;


db.myCollection.find().forEach(function(document) { 
    fieldToPrune = document.field_to_prune; 
    var unsetOp = {}; 
    for (var key in fieldToPrune) {     
        if ((wantedFields.indexOf(key) === -1) && Object.prototype.hasOwnProperty.call(fieldToPrune, key ) ) {  
            unsetOp["field_to_prune."+key] = " ";             
        } 
    } 
    bulk.find({ "_id": document._id }).updateOne( { "$unset": unsetOp } );         
    count++; 
    if (count % 1000 === 0) {
        // Execute per 1000 operations and re-init     
        bulk.execute();     
        bulk =  db.myCollection.initializeUnorderedBulkOp(); 
    } 
})

// Clean up queues
if (count > 0) { 
    bulk.execute(); 
}