The $concatArrays is used to concatenate arrays to return the concatenated array.
Let us create a collection with documents −
> db.demo338.insertOne({"Name":"Chris","Marks1":[ [56,67,45],[67,89,90,91]]}); { "acknowledged" : true, "insertedId" : ObjectId("5e5299baf8647eb59e56209f") }
Display all documents from a collection with the help of find() method −
> db.demo338.find().pretty();
This will produce the following output −
{ "_id" : ObjectId("5e5299baf8647eb59e56209f"), "Name" : "Chris", "Marks1" : [ [ 56, 67, 45 ], [ 67, 89, 90, 91 ] ] }
Following is the query to work on the existing multi-array field and concatenate arrays −
> db.demo338.aggregate([ ... { "$project": { ... "Marks": { ... "$reduce": { ... "input": "$Marks1", ... "initialValue": [], ... "in": { "$concatArrays": ["$$this", "$$value"] } ... } ... } ... }} ... ])
This will produce the following output &minus'
{ "_id" : ObjectId("5e5299baf8647eb59e56209f"), "Marks" : [ 67, 89, 90, 91, 56, 67, 45 ] }