Add a new element to an array of sub-documents. Surprising results observed

Question

Add a new element to an array of sub-documents. Surprising results observed

I have been attempting to add a new field to each sub-document within all arrays of sub-documents. My script is partially working, but instead of inserting the ordinal_number into every sub-document, it seems to only be adding it to the first sub-document in each comments array in the collection.

db.posts.find({
"comments.ordinal_number":{"$exists":true}}).forEach(function(data){
   for(var i = 0; i < data.comments.length; i++) {
     db.posts.update(
    { 
         "_id": data._id, 
         "comments.body": data.comments[i].body
     },
     {
         "$set": {
           "comments.$.ordinal_number":
               1
         }
     },true,true
    );
  }
});

output result:

    "link" : "cxzdzjkztkqraoqlgcru",
        "author" : "machine",
        "title" : "arbitrary title",
        "comments" : [
            {
                "body" : "...",
                "email" : "<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="d48ebb869bbda68c9a94a0bc819ab983b98dfab7bbb9">[email protected]</a>",
                "author" : "Foo bar",
                "ordinal_number" : 1
            },
            {
                "body" : "...",
                "email" : "<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="5431150d200504322e143f020e171e3a31227a373b39">[email protected]</a>",
                "author" : "Foo baz"
            }
]

javascript mongodb mongodb-query

Answer 1

Answer №1

To achieve maximum efficiency, it is essential to loop through the cursor and array entries while utilizing the $ operator to update each sub-document in the array using "bulk" operations.


var bulk = db.posts.initializeOrderedBulkOp();
var count = 0;
db.posts.find().forEach(function(doc) { 
    var nComments = doc.comments.length; 
    for (var i = 0; i < nComments; i++) {
        bulk.find( { 
            '_id': doc._id, 
            'comments': { '$elemMatch': { 'email': doc.comments[i]['email'] } }
        } ).update({
            '$set': { 'comments.$.ordinal_number': 1 } 
        }) 
    } 
    count++;
    if(count % 200 === 0) {   
        // Execute per 200 operations and re-init
        bulk.execute();     
        bulk = db.posts.initializeOrderedBulkOp(); 
     }
})

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

It's important to note that the Bulk API was introduced in version 2.6. If you are working on an older version, you will need to utilize the .update() method.


db.posts.find().forEach(function(doc) { 
    var nComments = doc.comments.length; 
    for (var i = 0; i < nComments; i++) {
        db.posts.update( 
            { 
                '_id': doc._id, 
                 'comments': { '$elemMatch': { 'email': doc.comments[i]['email'] } }
            }, 

            { '$set': { 'comments.$.ordinal_number': 1 } }
         ) 
    } 
})

Answer 2

To achieve maximum efficiency, it is essential to loop through the cursor and array entries while utilizing the $ operator to update each sub-document in the array using "bulk" operations.


var bulk = db.posts.initializeOrderedBulkOp();
var count = 0;
db.posts.find().forEach(function(doc) { 
    var nComments = doc.comments.length; 
    for (var i = 0; i < nComments; i++) {
        bulk.find( { 
            '_id': doc._id, 
            'comments': { '$elemMatch': { 'email': doc.comments[i]['email'] } }
        } ).update({
            '$set': { 'comments.$.ordinal_number': 1 } 
        }) 
    } 
    count++;
    if(count % 200 === 0) {   
        // Execute per 200 operations and re-init
        bulk.execute();     
        bulk = db.posts.initializeOrderedBulkOp(); 
     }
})

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

It's important to note that the Bulk API was introduced in version 2.6. If you are working on an older version, you will need to utilize the .update() method.


db.posts.find().forEach(function(doc) { 
    var nComments = doc.comments.length; 
    for (var i = 0; i < nComments; i++) {
        db.posts.update( 
            { 
                '_id': doc._id, 
                 'comments': { '$elemMatch': { 'email': doc.comments[i]['email'] } }
            }, 

            { '$set': { 'comments.$.ordinal_number': 1 } }
         ) 
    } 
})

Add a new element to an array of sub-documents. Surprising results observed

Answer №1

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