I have a document with a field containing a very long string. I need to concatenate another string to the end of the string already contained in the field.

The way I do it now is that, from Java, I fetch the document, extract the string in the field, append the string to the end and finally update the document with the new string.

The problem: The string contained in the field is very long, which means that it takes time and resources to retrieve and work with this string in Java. Furthermore, this is an operation that is done several times per second.

My question: Is there a way to concatenate a string to an existing field, without having to fetch (db.<doc>.find()) the contents of the field first? In reality all I want is (field.contents += new_string).

I already made this work using Javascript and eval, but as I found out, MongoDB locks the database when it executes javascript, which makes the overall application even slower.

Solution 1

Starting Mongo 4.2, db.collection.update() can accept an aggregation pipeline, finally allowing the update of a field based on its current value:

// { a: "Hello" }
  [{ $set: { a: { $concat: [ "$a", "World" ] } } }],
  { multi: true }
// { a: "HelloWorld" }
  • The first part {} is the match query, filtering which documents to update (in this case all documents).

  • The second part [{ $set: { a: { $concat: [ "$a", "World" ] } } }] is the update aggregation pipeline (note the squared brackets signifying the use of an aggregation pipeline). $set (alias of $addFields) is a new aggregation operator which in this case replaces the field's value (by concatenating a itself with the suffix "World"). Note how a is modified directly based on its own value ($a).

  • Don't forget { multi: true }, otherwise only the first matching document will be updated.

Solution 2

For example (it's append to the start, the same story ):


{ "_id" : ObjectId("56993251e843bb7e0447829d"), "name" : "London City", "city" : "London" }

   .find( { $text: { $search: "City" } })
       function(e, i){ 
           e.name='Big ' + e.name; 


{ "_id" : ObjectId("56993251e843bb7e0447829d"), "name" : "Big London City", "city" : "London" }

Solution 3

Old topic but i had the same problem. Since mongo 2.4, you can use $concat from aggregation framework.


Consider these documents :

    "_id" : ObjectId("5941003d5e785b5c0b2ac78d"),
    "title" : "cov"

    "_id" : ObjectId("594109b45e785b5c0b2ac97d"),
    "title" : "fefe"

Append fefe to title field :

        { $project: { title: { $concat: [ "$title", "fefe"] } } }

The result of aggregation will be :

    "_id" : ObjectId("5941003d5e785b5c0b2ac78d"),
    "title" : "covfefe"

    "_id" : ObjectId("594109b45e785b5c0b2ac97d"),
    "title" : "fefefefe"

You can then save the results with a bulk, see this answer for that.

Solution 4

this is a sample of one document i have :

    "_id" : 1,
    "s" : 1,
    "ser" : 2,
    "p" : "9919871172",
    "d" : ISODate("2018-05-30T05:00:38.057Z"),
    "per" : "10"

to append a string to any feild you can run a forEach loop throught all documents and then update desired field:


Solution 5

This would not be possible. One optimization you can do is create batches of updates. i.e. fetch 10K documents, append relevant strings to each of their keys, and then save them as single batch. Most mongodb drivers support batch operations.

Solution 6

    // query 

    // update 
        $set: {<field>:this.<field>+"<new string>"}

    // options 
        "multi" : true,  // update only one document 
        "upsert" : false  // insert a new document, if no existing document match the query