Append a string to the end of an existing field in MongoDB

Question

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

Answer 1

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.

Answer 2

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" }
db.collection.update(
  {},
  [{ $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.

Answer 3

db.getCollection('<collection>').update(
    // 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 
    });

Answer 4

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

before

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

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

after:

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

Answer 5

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:

db.getCollection('jafar').find({}).forEach(function(el){
    db.getCollection('jafar').update(
        {p:el.p},
        {$set:{p:'98'+el.p}})    
    })

Answer 6

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

Example

Consider these documents :

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

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

Append fefe to title field :

db.getCollection('test_append_string').aggregate(
    [
        { $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.