// unwind & bring the internal array to root level

db.collection.aggregate([

    {

     $lookup: {

          localField: "_id",

          from: "foreignCollection",

          foreignField: "foreign_id",

          as: "internalArray",

    },

  { $unwind: "$internalArray" },

  { "$replaceRoot": { "newRoot": "$internalArray" } }

])