$concat (expression operator)

Definition

$concat

Concatenates strings and returns the concatenated string.

$concat has this syntax:

{ $concat: [ <expression1>, <expression2>, ... ] }

The arguments can be any valid expression as long as they resolve to strings. For more information on expressions, see Expressions.

If the argument resolves to a value of null or refers to a field that is missing, $concat returns null.

Example

The following example uses the movies collection from the sample_mflix sample dataset. For details on how to load this dataset into your deployment, see Load the sample dataset.

Use the $concat operator to concatenate the title field and the rated field with a " - " delimiter. The runtime filter limits the result to a small, representative set of documents:

db.movies.aggregate( [
   { $match: { runtime: { $gt: 1000 } } },
   {
      $project: {
         _id: 0,
         title: 1,
         titleRated: { $concat: [ "$title", " - ", "$rated" ] }
      }
   },
   { $sort: { title: 1 } }
] )

[
  {
    title: 'Baseball',
    titleRated: 'Baseball - TV-PG'
  },
  {
    title: 'Centennial',
    titleRated: null
  }
]