Jane wants to create two collections in MongoDB called the parent collection and the child collection. The relationship between the two collections is one-to-many. One document in the parent collection has many related documents in the child collection. She has written the following aggregation query.

db.parent.aggregate([

            {$unwind:"$myArray"},

            {$lookup:

                  { from: "child", 

                        localField: "myArray",

                        foreignField: "_id",   

                        as: "myArrayDetail"}   

                  }])

What kind of join is $lookup?