Join mongo db - with lookup

join.js

db.applications.aggregate([
    { "$unwind": "$servers" },
    { "$lookup": {
       "from": "servers",
       "localField": "servers",
       "foreignField": "_id",
       "as": "servers_details"
    }},
    // Unwind the result arrays ( likely one or none )
    { "$unwind": "$servers_details" },
    // Group back to arrays
    { "$group": {
        "_id": "$_id",
        "servers": { "$push": "$servers" },
        "servers_details": { "$push": "$servers_details" }
    }}
])