$concat (aggregation)
Definition
$concat
has the following syntax:
- { $concat: [ <expression1>, <expression2>, ... ] }
The arguments can be any valid expression as long as they resolve tostrings. For more information on expressions, seeExpressions.
If the argument resolves to a value of null
or refers to a fieldthat is missing, $concat
returns null
.
Examples
Consider a inventory
collection with the following documents:
- { "_id" : 1, "item" : "ABC1", quarter: "13Q1", "description" : "product 1" }
- { "_id" : 2, "item" : "ABC2", quarter: "13Q4", "description" : "product 2" }
- { "_id" : 3, "item" : "XYZ1", quarter: "14Q2", "description" : null }
The following operation uses the $concat
operator toconcatenate the item
field and the description
field with a” - ” delimiter.
- db.inventory.aggregate(
- [
- { $project: { itemDescription: { $concat: [ "$item", " - ", "$description" ] } } }
- ]
- )
The operation returns the following results:
- { "_id" : 1, "itemDescription" : "ABC1 - product 1" }
- { "_id" : 2, "itemDescription" : "ABC2 - product 2" }
- { "_id" : 3, "itemDescription" : null }