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MongoDB $graphLookup inside update query [duplicate]

In MongoDB, is it possible to update the value of a field using the value from another field? The equivalent SQL would be something like:
UPDATE Person SET Name = FirstName + ' ' + LastName
And the MongoDB pseudo-code would be:
db.person.update( {}, { $set : { name : firstName + ' ' + lastName } );

The best way to do this is in version 4.2+ which allows using the aggregation pipeline in the update document and the updateOne, updateMany, or update(deprecated in most if not all languages drivers) collection methods.
MongoDB 4.2+
Version 4.2 also introduced the $set pipeline stage operator, which is an alias for $addFields. I will use $set here as it maps with what we are trying to achieve.
db.collection.<update method>(
{},
[
{"$set": {"name": { "$concat": ["$firstName", " ", "$lastName"]}}}
]
)
Note that square brackets in the second argument to the method specify an aggregation pipeline instead of a plain update document because using a simple document will not work correctly.
MongoDB 3.4+
In 3.4+, you can use $addFields and the $out aggregation pipeline operators.
db.collection.aggregate(
[
{ "$addFields": {
"name": { "$concat": [ "$firstName", " ", "$lastName" ] }
}},
{ "$out": <output collection name> }
]
)
Note that this does not update your collection but instead replaces the existing collection or creates a new one. Also, for update operations that require "typecasting", you will need client-side processing, and depending on the operation, you may need to use the find() method instead of the .aggreate() method.
MongoDB 3.2 and 3.0
The way we do this is by $projecting our documents and using the $concat string aggregation operator to return the concatenated string.
You then iterate the cursor and use the $set update operator to add the new field to your documents using bulk operations for maximum efficiency.
Aggregation query:
var cursor = db.collection.aggregate([
{ "$project": {
"name": { "$concat": [ "$firstName", " ", "$lastName" ] }
}}
])
MongoDB 3.2 or newer
You need to use the bulkWrite method.
var requests = [];
cursor.forEach(document => {
requests.push( {
'updateOne': {
'filter': { '_id': document._id },
'update': { '$set': { 'name': document.name } }
}
});
if (requests.length === 500) {
//Execute per 500 operations and re-init
db.collection.bulkWrite(requests);
requests = [];
}
});
if(requests.length > 0) {
db.collection.bulkWrite(requests);
}
MongoDB 2.6 and 3.0
From this version, you need to use the now deprecated Bulk API and its associated methods.
var bulk = db.collection.initializeUnorderedBulkOp();
var count = 0;
cursor.snapshot().forEach(function(document) {
bulk.find({ '_id': document._id }).updateOne( {
'$set': { 'name': document.name }
});
count++;
if(count%500 === 0) {
// Excecute per 500 operations and re-init
bulk.execute();
bulk = db.collection.initializeUnorderedBulkOp();
}
})
// clean up queues
if(count > 0) {
bulk.execute();
}
MongoDB 2.4
cursor["result"].forEach(function(document) {
db.collection.update(
{ "_id": document._id },
{ "$set": { "name": document.name } }
);
})

You should iterate through. For your specific case:
db.person.find().snapshot().forEach(
function (elem) {
db.person.update(
{
_id: elem._id
},
{
$set: {
name: elem.firstname + ' ' + elem.lastname
}
}
);
}
);

Apparently there is a way to do this efficiently since MongoDB 3.4, see styvane's answer.
Obsolete answer below
You cannot refer to the document itself in an update (yet). You'll need to iterate through the documents and update each document using a function. See this answer for an example, or this one for server-side eval().

For a database with high activity, you may run into issues where your updates affect actively changing records and for this reason I recommend using snapshot()
db.person.find().snapshot().forEach( function (hombre) {
hombre.name = hombre.firstName + ' ' + hombre.lastName;
db.person.save(hombre);
});
http://docs.mongodb.org/manual/reference/method/cursor.snapshot/

Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update/creation of a field based on another field:
// { firstName: "Hello", lastName: "World" }
db.collection.updateMany(
{},
[{ $set: { name: { $concat: [ "$firstName", " ", "$lastName" ] } } }]
)
// { "firstName" : "Hello", "lastName" : "World", "name" : "Hello World" }
The first part {} is the match query, filtering which documents to update (in our case all documents).
The second part [{ $set: { name: { ... } }] is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline). $set is a new aggregation operator and an alias of $addFields.

Regarding this answer, the snapshot function is deprecated in version 3.6, according to this update. So, on version 3.6 and above, it is possible to perform the operation this way:
db.person.find().forEach(
function (elem) {
db.person.update(
{
_id: elem._id
},
{
$set: {
name: elem.firstname + ' ' + elem.lastname
}
}
);
}
);

I tried the above solution but I found it unsuitable for large amounts of data. I then discovered the stream feature:
MongoClient.connect("...", function(err, db){
var c = db.collection('yourCollection');
var s = c.find({/* your query */}).stream();
s.on('data', function(doc){
c.update({_id: doc._id}, {$set: {name : doc.firstName + ' ' + doc.lastName}}, function(err, result) { /* result == true? */} }
});
s.on('end', function(){
// stream can end before all your updates do if you have a lot
})
})

update() method takes aggregation pipeline as parameter like
db.collection_name.update(
{
// Query
},
[
// Aggregation pipeline
{ "$set": { "id": "$_id" } }
],
{
// Options
"multi": true // false when a single doc has to be updated
}
)
The field can be set or unset with existing values using the aggregation pipeline.
Note: use $ with field name to specify the field which has to be read.

Here's what we came up with for copying one field to another for ~150_000 records. It took about 6 minutes, but is still significantly less resource intensive than it would have been to instantiate and iterate over the same number of ruby objects.
js_query = %({
$or : [
{
'settings.mobile_notifications' : { $exists : false },
'settings.mobile_admin_notifications' : { $exists : false }
}
]
})
js_for_each = %(function(user) {
if (!user.settings.hasOwnProperty('mobile_notifications')) {
user.settings.mobile_notifications = user.settings.email_notifications;
}
if (!user.settings.hasOwnProperty('mobile_admin_notifications')) {
user.settings.mobile_admin_notifications = user.settings.email_admin_notifications;
}
db.users.save(user);
})
js = "db.users.find(#{js_query}).forEach(#{js_for_each});"
Mongoid::Sessions.default.command('$eval' => js)

With MongoDB version 4.2+, updates are more flexible as it allows the use of aggregation pipeline in its update, updateOne and updateMany. You can now transform your documents using the aggregation operators then update without the need to explicity state the $set command (instead we use $replaceRoot: {newRoot: "$$ROOT"})
Here we use the aggregate query to extract the timestamp from MongoDB's ObjectID "_id" field and update the documents (I am not an expert in SQL but I think SQL does not provide any auto generated ObjectID that has timestamp to it, you would have to automatically create that date)
var collection = "person"
agg_query = [
{
"$addFields" : {
"_last_updated" : {
"$toDate" : "$_id"
}
}
},
{
$replaceRoot: {
newRoot: "$$ROOT"
}
}
]
db.getCollection(collection).updateMany({}, agg_query, {upsert: true})

(I would have posted this as a comment, but couldn't)
For anyone who lands here trying to update one field using another in the document with the c# driver...
I could not figure out how to use any of the UpdateXXX methods and their associated overloads since they take an UpdateDefinition as an argument.
// we want to set Prop1 to Prop2
class Foo { public string Prop1 { get; set; } public string Prop2 { get; set;} }
void Test()
{
var update = new UpdateDefinitionBuilder<Foo>();
update.Set(x => x.Prop1, <new value; no way to get a hold of the object that I can find>)
}
As a workaround, I found that you can use the RunCommand method on an IMongoDatabase (https://docs.mongodb.com/manual/reference/command/update/#dbcmd.update).
var command = new BsonDocument
{
{ "update", "CollectionToUpdate" },
{ "updates", new BsonArray
{
new BsonDocument
{
// Any filter; here the check is if Prop1 does not exist
{ "q", new BsonDocument{ ["Prop1"] = new BsonDocument("$exists", false) }},
// set it to the value of Prop2
{ "u", new BsonArray { new BsonDocument { ["$set"] = new BsonDocument("Prop1", "$Prop2") }}},
{ "multi", true }
}
}
}
};
database.RunCommand<BsonDocument>(command);

MongoDB 4.2+ Golang
result, err := collection.UpdateMany(ctx, bson.M{},
mongo.Pipeline{
bson.D{{"$set",
bson.M{"name": bson.M{"$concat": []string{"$lastName", " ", "$firstName"}}}
}},
)

If the specific field is double, convert it to Int [duplicate]

I am trying to change the type of a field from within the mongo shell.
I am doing this...
db.meta.update(
{'fields.properties.default': { $type : 1 }},
{'fields.properties.default': { $type : 2 }}
)
But it's not working!

The only way to change the $type of the data is to perform an update on the data where the data has the correct type.
In this case, it looks like you're trying to change the $type from 1 (double) to 2 (string).
So simply load the document from the DB, perform the cast (new String(x)) and then save the document again.
If you need to do this programmatically and entirely from the shell, you can use the find(...).forEach(function(x) {}) syntax.
In response to the second comment below. Change the field bad from a number to a string in collection foo.
db.foo.find( { 'bad' : { $type : 1 } } ).forEach( function (x) {
x.bad = new String(x.bad); // convert field to string
db.foo.save(x);
});

Convert String field to Integer:
db.db-name.find({field-name: {$exists: true}}).forEach(function(obj) {
obj.field-name = new NumberInt(obj.field-name);
db.db-name.save(obj);
});
Convert Integer field to String:
db.db-name.find({field-name: {$exists: true}}).forEach(function(obj) {
obj.field-name = "" + obj.field-name;
db.db-name.save(obj);
});

Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update of a field based on its own value:
// { a: "45", b: "x" }
// { a: 53, b: "y" }
db.collection.updateMany(
{ a : { $type: 1 } },
[{ $set: { a: { $toString: "$a" } } }]
)
// { a: "45", b: "x" }
// { a: "53", b: "y" }
The first part { a : { $type: 1 } } is the match query:
It filters which documents to update.
In this case, since we want to convert "a" to string when its value is a double, this matches elements for which "a" is of type 1 (double)).
This table provides the code representing the different possible types.
The second part [{ $set: { a: { $toString: "$a" } } }] is the update aggregation pipeline:
Note the squared brackets signifying that this update query uses an aggregation pipeline.
$set is a new aggregation operator (Mongo 4.2) which in this case modifies a field.
This can be simply read as "$set" the value of "a" to "$a" converted "$toString".
What's really new here, is being able in Mongo 4.2 to reference the document itself when updating it: the new value for "a" is based on the existing value of "$a".
Also note "$toString" which is a new aggregation operator introduced in Mongo 4.0.
In case your cast isn't from double to string, you have the choice between different conversion operators introduced in Mongo 4.0 such as $toBool, $toInt, ...
And if there isn't a dedicated converter for your targeted type, you can replace { $toString: "$a" } with a $convert operation: { $convert: { input: "$a", to: 2 } } where the value for to can be found in this table:
db.collection.updateMany(
{ a : { $type: 1 } },
[{ $set: { a: { $convert: { input: "$a", to: 2 } } } }]
)

For string to int conversion.
db.my_collection.find().forEach( function(obj) {
obj.my_value= new NumberInt(obj.my_value);
db.my_collection.save(obj);
});
For string to double conversion.
obj.my_value= parseInt(obj.my_value, 10);
For float:
obj.my_value= parseFloat(obj.my_value);

db.coll.find().forEach(function(data) {
db.coll.update({_id:data._id},{$set:{myfield:parseInt(data.myfield)}});
})

all answers so far use some version of forEach, iterating over all collection elements client-side.
However, you could use MongoDB's server-side processing by using aggregate pipeline and $out stage as :
the $out stage atomically replaces the existing collection with the
new results collection.
example:
db.documents.aggregate([
{
$project: {
_id: 1,
numberField: { $substr: ['$numberField', 0, -1] },
otherField: 1,
differentField: 1,
anotherfield: 1,
needolistAllFieldsHere: 1
},
},
{
$out: 'documents',
},
]);

To convert a field of string type to date field, you would need to iterate the cursor returned by the find() method using the forEach() method, within the loop convert the field to a Date object and then update the field using the $set operator.
Take advantage of using the Bulk API for bulk updates which offer better performance as you will be sending the operations to the server in batches of say 1000 which gives you a better performance as you are not sending every request to the server, just once in every 1000 requests.
The following demonstrates this approach, the first example uses the Bulk API available in MongoDB versions >= 2.6 and < 3.2. It updates all
the documents in the collection by changing all the created_at fields to date fields:
var bulk = db.collection.initializeUnorderedBulkOp(),
counter = 0;
db.collection.find({"created_at": {"$exists": true, "$type": 2 }}).forEach(function (doc) {
var newDate = new Date(doc.created_at);
bulk.find({ "_id": doc._id }).updateOne({
"$set": { "created_at": newDate}
});
counter++;
if (counter % 1000 == 0) {
bulk.execute(); // Execute per 1000 operations and re-initialize every 1000 update statements
bulk = db.collection.initializeUnorderedBulkOp();
}
})
// Clean up remaining operations in queue
if (counter % 1000 != 0) { bulk.execute(); }
The next example applies to the new MongoDB version 3.2 which has since deprecated the Bulk API and provided a newer set of apis using bulkWrite():
var bulkOps = [];
db.collection.find({"created_at": {"$exists": true, "$type": 2 }}).forEach(function (doc) {
var newDate = new Date(doc.created_at);
bulkOps.push(
{
"updateOne": {
"filter": { "_id": doc._id } ,
"update": { "$set": { "created_at": newDate } }
}
}
);
})
db.collection.bulkWrite(bulkOps, { "ordered": true });

To convert int32 to string in mongo without creating an array just add "" to your number :-)
db.foo.find( { 'mynum' : { $type : 16 } } ).forEach( function (x) {
x.mynum = x.mynum + ""; // convert int32 to string
db.foo.save(x);
});

What really helped me to change the type of the object in MondoDB was just this simple line, perhaps mentioned before here...:
db.Users.find({age: {$exists: true}}).forEach(function(obj) {
obj.age = new NumberInt(obj.age);
db.Users.save(obj);
});
Users are my collection and age is the object which had a string instead of an integer (int32).

You can easily convert the string data type to numerical data type.
Don't forget to change collectionName & FieldName.
for ex : CollectionNmae : Users & FieldName : Contactno.
Try this query..
db.collectionName.find().forEach( function (x) {
x.FieldName = parseInt(x.FieldName);
db.collectionName.save(x);
});

I need to change datatype of multiple fields in the collection, so I used the following to make multiple data type changes in the collection of documents. Answer to an old question but may be helpful for others.
db.mycoll.find().forEach(function(obj) {
if (obj.hasOwnProperty('phone')) {
obj.phone = "" + obj.phone; // int or longint to string
}
if (obj.hasOwnProperty('field-name')) {
obj.field-name = new NumberInt(obj.field-name); //string to integer
}
if (obj.hasOwnProperty('cdate')) {
obj.cdate = new ISODate(obj.cdate); //string to Date
}
db.mycoll.save(obj);
});

demo change type of field mid from string to mongo objectId using mongoose
Post.find({}, {mid: 1,_id:1}).exec(function (err, doc) {
doc.map((item, key) => {
Post.findByIdAndUpdate({_id:item._id},{$set:{mid: mongoose.Types.ObjectId(item.mid)}}).exec((err,res)=>{
if(err) throw err;
reply(res);
});
});
});
Mongo ObjectId is just another example of such styles as
Number, string, boolean that hope the answer will help someone else.

I use this script in mongodb console for string to float conversions...
db.documents.find({ 'fwtweaeeba' : {$exists : true}}).forEach( function(obj) {
obj.fwtweaeeba = parseFloat( obj.fwtweaeeba );
db.documents.save(obj); } );
db.documents.find({ 'versions.0.content.fwtweaeeba' : {$exists : true}}).forEach( function(obj) {
obj.versions[0].content.fwtweaeeba = parseFloat( obj.versions[0].content.fwtweaeeba );
db.documents.save(obj); } );
db.documents.find({ 'versions.1.content.fwtweaeeba' : {$exists : true}}).forEach( function(obj) {
obj.versions[1].content.fwtweaeeba = parseFloat( obj.versions[1].content.fwtweaeeba );
db.documents.save(obj); } );
db.documents.find({ 'versions.2.content.fwtweaeeba' : {$exists : true}}).forEach( function(obj) {
obj.versions[2].content.fwtweaeeba = parseFloat( obj.versions[2].content.fwtweaeeba );
db.documents.save(obj); } );
And this one in php)))
foreach($db->documents->find(array("type" => "chair")) as $document){
$db->documents->update(
array('_id' => $document[_id]),
array(
'$set' => array(
'versions.0.content.axdducvoxb' => (float)$document['versions'][0]['content']['axdducvoxb'],
'versions.1.content.axdducvoxb' => (float)$document['versions'][1]['content']['axdducvoxb'],
'versions.2.content.axdducvoxb' => (float)$document['versions'][2]['content']['axdducvoxb'],
'axdducvoxb' => (float)$document['axdducvoxb']
)
),
array('$multi' => true)
);
}

The above answers almost worked but had a few challenges-
Problem 1: db.collection.save no longer works in MongoDB 5.x
For this, I used replaceOne().
Problem 2: new String(x.bad) was giving exponential number
I used "" + x.bad as suggested above.
My version:
let count = 0;
db.user
.find({
custID: {$type: 1},
})
.forEach(function (record) {
count++;
const actualValue = record.custID;
record.custID = "" + record.custID;
console.log(`${count}. Updating User(id:${record._id}) from old id [${actualValue}](${typeof actualValue}) to [${record.custID}](${typeof record.custID})`)
db.user.replaceOne({_id: record._id}, record);
});
And for millions of records, here are the output (for future investigation/reference)-

How to find longest and shortest length of a value for a field in mongoDb?

The data type of the field is String. I would like to find the length of the longest and shortest value for a field in mongoDB.
I have totally 500000 documents in my collection.

In modern releases MongoDB has the $strLenBytes or $strLenCP aggregation operators than allow you to simply do:
Class.collection.aggregate([
{ "$group" => {
"_id" => nil,
"max" => { "$max" => { "$strLenCP" => "$a" } },
"min" => { "$min" => { "$strLenCP" => "$a" } }
}}
])
Where "a" is the string property in your document you want to get the min and max length from.
To output the minimum and maximum length, the best approach available is to use mapReduce with a few tricks to just keep the values.
First you define a mapper function which is just really going to output a single item from your collection to reduce the load:
map = Q%{
function () {
if ( this.a.length < store[0] )
store[0] = this.a.length;
if ( this.a.length > store[1] )
store[1] = this.a.length;
if ( count == 0 )
emit( null, 0 );
count++;
}
}
Since this is working mostly with a globally scoped variable keeping the min and max lengths you just want to substitute this in a finalize function on the single document emitted. There is no reduce stage, but define a "blank" function for this even though it is not called:
reduce = Q%{ function() {} }
finalize = Q%{
function(key,value) {
return {
min: store[0],
max: store[1]
};
}
}
Then call the mapReduce operation:
Class.map_reduce(map,reduce).out(inline: 1).finalize(finalize).scope(store: [], count: 0)
So all the work is done on the server and not by iterating results sent to the client application. On a small set like this:
{ "_id" : ObjectId("543e8ee7ddd272814f919472"), "a" : "this" }
{ "_id" : ObjectId("543e8eedddd272814f919473"), "a" : "something" }
{ "_id" : ObjectId("543e8ef6ddd272814f919474"), "a" : "other" }
You get a result like this (shell output, but much the same for the driver ):
{
"results" : [
{
"_id" : null,
"value" : {
"min" : 4,
"max" : 9
}
}
],
"timeMillis" : 1,
"counts" : {
"input" : 3,
"emit" : 1,
"reduce" : 0,
"output" : 1
},
"ok" : 1
}
So mapReduce allows the JavaScript processing on the server to do this fairly quickly, reducing your network traffic. There is no other native way at present for MongoDB to return a string length right now, so the JavaScript processing is necessary on the server.

You can use a mongo shell script. Note that it will perform a full table scan.
function findMinMax() {
var max = 0;
var min = db.collection.findOne().fieldName.length;
db.collection.find().forEach(function(doc) {
var currentLength = doc.fieldName.length;
if (currentLength > max) {
max = currentLength;
}
if (currentLength < min) {
min = currentLength;
}
});
print(max);
print(min);
}
use <databaseName>
findMinMax();
You can save the function in a file say c:\minMax.js and run the file as,
c:\mongodb\bin> mongo dbName < c:\minMax.js
Note: you may need to supply the necessary hostname, user name, password to connect to your database.
c:\mongodb\bin> mongo --host hostName --port portNumber -u userName -p password dbName < c:\minMax.js

For getting the longest value for a field
db.entities.aggregate([{ $match:{ condition } },{
$addFields: {
"length": { $strLenCP: "$feildName" }
}},
{ "$sort": { "length": -1 } },
{$limit:1}
])
Change the { "$sort": { "length": -1 } } to { "$sort": { "length": 1 } } for the shortest value for a field

Merge and aggregate some fields of two or more collections with identical schema mongodb

I have some collections with identical schema and I want to perform a merge + aggregation on them. The schemas are simple and look like this:
{ 'fr': 1, 'to': 1, 'wg': 213}
{ 'fr': 1, 'to': 2, 'wg': 53}
{ 'fr': 2, 'to': 2, 'wg': 5521}
The following code works for merging two collections, but I am wondering if there is a faster solutions and/or one that could merge multiple collections in a similar way without creating nested calls:
var c = db.collection('first').find()
c.each(function(err, doc) {
if (err) throw err
if (doc == null) {
console.log('done')
return
}
db.collection('second').findOne({
'fr': doc['fr'],
'to': doc['to']
}, function(err, doc2) {
if (err) throw err
db.collection('my_results').save({
'fr': doc['fr'],
'to': doc['to'],
'wg': doc['wg'] + doc2['wg']
}, function(err) {
if (err) throw err
})
})
})

There are no absolute free operations here since you cannot do joins with MongoDB. But you can get the output you want using mapReduce and some of its features.
So first create a mapper:
var mapper = function () {
emit( { fr: this.fr, to: this.to }, this.wg )
};
And then a reducer:
var reducer = function (key,values) {
return Array.sum( values );
};
Then you run the mapReduce operation with the output set to a different collection:
db.first.mapReduce(mapper,reducer,{ "out": { "reduce": "third" } })
Note the "out" options there which are explained in this manual section. The point is, despite possibly misleading statistics output in the console, that "reduce" statement is very important. This is so when we run the same code against the other collection:
db.second.mapReduce(mapper,reducer,{ "out": { "reduce": "third" } })
What actually happens in the result, is the output from the first operation is also passed into the "reduce" phase of the second operation.
The end result is that all the values from both collections with the same key values will be added together in the "third" collection:
{ "_id" : { "fr" : 1, "to" : 1 }, "value" : 426 }
{ "_id" : { "fr" : 1, "to" : 2 }, "value" : 106 }
{ "_id" : { "fr" : 2, "to" : 2 }, "value" : 11042 }
You can make that a little fancier if you wanted your fr and to to be the unique combination of two possibles in either order, or even run another mapReduce or aggregate over those results.

How to replace string in all documents in Mongo

I need to replace a string in certain documents. I have googled this code, but it unfortunately does not change anything. I am not sure about the syntax on the line bellow:
pulpdb = db.getSisterDB("pulp_database");
var cursor = pulpdb.repos.find();
while (cursor.hasNext()) {
var x = cursor.next();
x['source']['url'].replace('aaa', 'bbb'); // is this correct?
db.foo.update({_id : x._id}, x);
}
I would like to add some debug prints to see what the value is, but I have no experience with MongoDB Shell. I just need to replace this:
{ "source": { "url": "http://aaa/xxx/yyy" } }
with
{ "source": { "url": "http://bbb/xxx/yyy" } }

It doesn't correct generally: if you have string http://aaa/xxx/aaa (yyy equals to aaa) you'll end up with http://bbb/xxx/bbb.
But if you ok with this, code will work.
To add debug info use print function:
var cursor = db.test.find();
while (cursor.hasNext()) {
var x = cursor.next();
print("Before: "+x['source']['url']);
x['source']['url'] = x['source']['url'].replace('aaa', 'bbb');
print("After: "+x['source']['url']);
db.test.update({_id : x._id}, x);
}
(And by the way, if you want to print out objects, there is also printjson function)

The best way to do this if you are on MongoDB 2.6 or newer is looping over the cursor object using the .forEach method and update each document usin "bulk" operations for maximum efficiency.
var bulk = db.collection.initializeOrderedBulkOp();
var count = 0;
db.collection.find().forEach(function(doc) {
print("Before: "+doc.source.url);
bulk.find({ '_id': doc._id }).update({
'$set': { 'source.url': doc.source.url.replace('aaa', 'bbb') }
})
count++;
if(count % 200 === 0) {
bulk.execute();
bulk = db.collection.initializeOrderedBulkOp();
}
// Clean up queues
if (count > 0)
bulk.execute();
From MongoDB 3.2 the Bulk() API and its associated methods are deprecated you will need to use the db.collection.bulkWrite() method.
You will need loop over the cursor, build your query dynamically and $push each operation to an array.
var operations = [];
db.collection.find().forEach(function(doc) {
print("Before: "+doc.source.url);
var operation = {
updateOne: {
filter: { '_id': doc._id },
update: {
'$set': { 'source.url': doc.source.url.replace('aaa', 'bbb') }
}
}
};
operations.push(operation);
})
operations.push({
ordered: true,
writeConcern: { w: "majority", wtimeout: 5000 }
})
db.collection.bulkWrite(operations);

Nowadays,
starting Mongo 4.2, db.collection.updateMany (alias of db.collection.update) can accept an aggregation pipeline, finally allowing the update of a field based on its own value.
starting Mongo 4.4, the new aggregation operator $replaceOne makes it very easy to replace part of a string.
// { "source" : { "url" : "http://aaa/xxx/yyy" } }
// { "source" : { "url" : "http://eee/xxx/yyy" } }
db.collection.updateMany(
{ "source.url": { $regex: /aaa/ } },
[{
$set: { "source.url": {
$replaceOne: { input: "$source.url", find: "aaa", replacement: "bbb" }
}}
}]
)
// { "source" : { "url" : "http://bbb/xxx/yyy" } }
// { "source" : { "url" : "http://eee/xxx/yyy" } }
The first part ({ "source.url": { $regex: /aaa/ } }) is the match query, filtering which documents to update (the ones containing "aaa")
The second part ($set: { "source.url": {...) is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline):
$set is a new aggregation operator (Mongo 4.2) which in this case replaces the value of a field.
The new value is computed with the new $replaceOne operator. Note how source.url is modified directly based on the its own value ($source.url).
Note that this is fully handled server side which won't allow you to perform the debug printing part of your question.

MongoDB can do string search/replace via mapreduce. Yes, you need to have a very special data structure for it -- you can't have anything in the top keys but you need to store everything under a subdocument under value. Like this:
{
"_id" : ObjectId("549dafb0a0d0ca4ed723e37f"),
"value" : {
"title" : "Top 'access denied' errors",
"parent" : "system.admin_reports",
"p" : "\u0001\u001a%"
}
}
Once you have this neatly set up you can do:
$map = new \MongoCode("function () {
this.value['p'] = this.value['p'].replace('$from', '$to');
emit(this._id, this.value);
}");
$collection = $this->mongoCollection();
// This won't be called.
$reduce = new \MongoCode("function () { }");
$collection_name = $collection->getName();
$collection->db->command([
'mapreduce' => $collection_name,
'map' => $map,
'reduce' => $reduce,
'out' => ['merge' => $collection_name],
'query' => $query,
'sort' => ['_id' => 1],
]);