Mongo convert embedded document to array

Question

Is there a way to convert a nested document structure into an array? Below is an example:

Input

"experience" : {
        "0" : {
            "duration" : "3 months",
            "end" : "August 2012",
            "organization" : {
                "0" : {
                    "name" : "Bank of China",
                    "profile_url" : "http://www.linkedin.com/company/13801"
                }
            },
            "start" : "June 2012",
            "title" : "Intern Analyst"
        }
    },

Expected Output:

"experience" : [
           {
            "duration" : "3 months",
            "end" : "August 2012",
            "organization" : {
                "0" : {
                    "name" : "Bank of China",
                    "profile_url" : "http://www.linkedin.com/company/13801"
                }
            },
            "start" : "June 2012",
            "title" : "Intern Analyst"
        }
    ],

Currently I am using a script to iterate over each element, convert them to an array & finally update the document. But it is taking a lot of time, is there a better way of doing this?

Answer 1

You still need to iterate over the content, but instead you should be writing back using bulk operations:

Either for MongoDB 2.6 and greater:

var bulk = db.collection.initializeUnorderedBulkOp(),
    count = 0;

db.collection.find({ 
   "$where": "return !Array.isArray(this.experience)"
}).forEach(function(doc) {
    bulk.find({ "_id": doc._id }).updateOne({
        "$set": { "experience": [doc.experience["0"]] }
    });
    count++;

    // Write once in 1000 entries
    if ( count % 1000 == 0 ) {
        bulk.execute();    
        bulk = db.collection.initializeUnorderedBulkOp();
    }
})

// Write the remaining
if ( count % 1000 != 0 )
    bulk.execute();

Or in modern releases of MongoDB 3.2 and greater, the bulkWrite() method is preferred:

var ops = [];

db.collection.find({ 
   "$where": "return !Array.isArray(this.experience)"
}).forEach(function(doc) {
   ops.push({
       "updateOne": {
           "filter": { "_id": doc._id },
           "update": { "$set": { "experience": [doc.experience["0"]] } }
       }
   });

   if ( ops.length == 1000 ) {
       db.collection.bulkWrite(ops,{ "ordered": false })
       ops = [];
   }
})

if ( ops.length > 0 )
    db.collection.bulkWrite(ops,{ "ordered": false });

So when writing back to the database over a cursor, then bulk write operations with "unordered" set is the way to go. It's only one write/response per batch of 1000 requests, which reduces a lot of overhead, and "unordered" means that writes can happen in parallel rather than in a serial order. It all makes it faster.

Answer 2

For mongoDB version >4.2 :

db.doc.aggregate([{ $match: {'experience.0': { $exists: false } } },
    {$project:{experience:["$experience.0"]}}, { $merge: { into: "doc", on: "_id" }
])

Note : Here we're merging the updated field/document with existing, but not replacing/updating entire document, default behavior of $merge is merge whenMatched document is found, You can pass other options like replace/keepExisting etc.

Ref: $merge