You can use the following syntax to concatenate strings from two fields into a new field in MongoDB:

db.myCollection.aggregate([
  { $project: { newfield: { $concat: [ "$field1", " - ", "$field2" ] } } },
  { $merge: "myCollection" }
])

This particular example concatenates the strings from “field1” and “field2” into a new field titled “newfield” and adds the new field to the collection titled myCollection.

The following example shows how to use this syntax in practice with a collection teams with the following documents:

db.teams.insertOne({team: "Mavs", conference: "Western", points: 31})
db.teams.insertOne({team: "Spurs", conference: "Western", points: 22})
db.teams.insertOne({team: "Rockets", conference: "Western", points: 19})
db.teams.insertOne({team: "Celtics", conference: "Eastern", points: 26})
db.teams.insertOne({team: "Cavs", conference: "Eastern", points: 33})
db.teams.insertOne({team: "Nets", conference: "Eastern", points: 38})

Example: Concatenate Strings in MongoDB

We can use the following code to concatenate the strings from the “team” field and the “conference” field into a new field titled “teamConf” and add this field to the teams collection:

db.teams.aggregate([
  { $project: { teamConf: { $concat: [ "$team", " - ", "$conference" ] } } },
  { $merge: "teams" }
])

Here’s what the updated collection now looks like:

{ _id: ObjectId("62013d8c4cb04b772fd7a90c"),
  team: 'Mavs',
  conference: 'Western',
  points: 31,
  teamConf: 'Mavs - Western' }
{ _id: ObjectId("62013d8c4cb04b772fd7a90d"),
  team: 'Spurs',
  conference: 'Western',
  points: 22,
  teamConf: 'Spurs - Western' }
{ _id: ObjectId("62013d8c4cb04b772fd7a90e"),
  team: 'Rockets',
  conference: 'Western',
  points: 19,
  teamConf: 'Rockets - Western' }
{ _id: ObjectId("62013d8c4cb04b772fd7a90f"),
  team: 'Celtics',
  conference: 'Eastern',
  points: 26,
  teamConf: 'Celtics - Eastern' }
{ _id: ObjectId("62013d8c4cb04b772fd7a910"),
  team: 'Cavs',
  conference: 'Eastern',
  points: 33,
  teamConf: 'Cavs - Eastern' }
{ _id: ObjectId("62013d8c4cb04b772fd7a911"),
  team: 'Nets',
  conference: 'Eastern',
  points: 38,
  teamConf: 'Nets - Eastern' } 

Notice that every document has a new field titled “teamConf” that contains the concatenation of the “team” and “conference” fields.

For this particular example we chose to concatenate the two strings together using a dash as a separator.

However, we could choose to concatenate the two strings without any separator value in between them.

The following code shows how to do so:

db.teams.aggregate([
  { $project: { teamConf: { $concat: [ "$team", "$conference" ] } } },
  { $merge: "teams" }
])

Here’s what the updated collection would look like:

{ _id: ObjectId("62013d8c4cb04b772fd7a90c"),
  team: 'Mavs',
  conference: 'Western',
  points: 31,
  teamConf: 'MavsWestern' }
{ _id: ObjectId("62013d8c4cb04b772fd7a90d"),
  team: 'Spurs',
  conference: 'Western',
  points: 22,
  teamConf: 'SpursWestern' }
{ _id: ObjectId("62013d8c4cb04b772fd7a90e"),
  team: 'Rockets',
  conference: 'Western',
  points: 19,
  teamConf: 'RocketWestern' }
{ _id: ObjectId("62013d8c4cb04b772fd7a90f"),
  team: 'Celtics',
  conference: 'Eastern',
  points: 26,
  teamConf: 'CelticsEastern' }
{ _id: ObjectId("62013d8c4cb04b772fd7a910"),
  team: 'Cavs',
  conference: 'Eastern',
  points: 33,
  teamConf: 'CavsEastern' }
{ _id: ObjectId("62013d8c4cb04b772fd7a911"),
  team: 'Nets',
  conference: 'Eastern',
  points: 38,
  teamConf: 'NetsEastern' } 

Notice that the new field titled “teamConf” contains the concatenation of the “team” and “conference” fields without any separator values between them.

Note: You can find the complete documentation for the $concat function here.

Additional Resources

The following tutorials explain how to perform other common operations in MongoDB:

MongoDB: How to Check if Field Contains a String
MongoDB: How to Add a New Field
MongoDB: How to Remove a Field