Mongo Db aggregate $lookup returns empty array

Question

when trying to "JOIN" operation with $lookup but results count is ok but "as" document is empty

I have two collections and i need to get user details from subscribercol with user_id in employer_jobscol

subscribercol

{
    "_id" : ObjectId("58187e7551d244640626d7e1"),
    "type" : "job_seeker",
    "firstname" : "vishnu",
    "lastname" : "kumar pv",
    "email_array" : {
        "primary" : "[email protected]",
        "secondary" : "[email protected]",
        "verified" : false
    },
    "address_array" : {
        "address" : "test address22d",
        "streetname" : "test222d",
        "pincode" : "test222d",
        "city" : "dddd",
        "state" : "ALASKA2d",
        "country" : "Argentinad"
    },
    "phone_array" : {
        "primary" : "",
        "secondary" : "",
        "verified" : ""
    },
    "languages" : [  
        "english", 
        "malayalam", 
        "english2"
    ]
}

employer_jobscol

{
    "_id" : ObjectId("582ada6b51d244073e2a7541"),
    "employer_id" : ObjectId("58187e7551d244640626d7e1"),
    "job_id" : "testjob16946",
    "job_title" : "Test Job 25",
    "category" : "IT",
    "vacancies" : "5",
    "salary" : "200000",
    "location" : "Kollam",
    "employer_name" : "test test",
    "mobile" : "9123456987",
    "video" : "",
    "image" : "",
    "work_place" : "option1",
    "email" : "[email protected]",
    "skills" : [ 
        "php"
    ],
    "isActive" : true,
    "applied_users" : [ 
        {
            "user_id" : ObjectId("581b364751d2445c311cf6f1"),
            "accepted" : false
        }, 
        {
            "user_id" : ObjectId("58187e7551d244640626d7e1"),
            "accepted" : false
        }
    ]
}

my database query here, (executed with Robomongo )

db.getCollection('employer_jobscol').aggregate([   {
      $unwind: "$applied_users"
   },
    {
      $lookup:
        {
          from: "subscribercol",
          localField: "user_id",
          foreignField: "_id",
          as: "subscribercol_docs"
        }
   } 
])

Result is

---

{
    "_id" : ObjectId("582ada6b51d244073e2a7541"),
    "employer_id" : ObjectId("58187e7551d244640626d7e1"),
    "job_id" : "testjob16946",
    "job_title" : "Test Job 25",
    "category" : "IT",
    "vacancies" : "5",
    "salary" : "200000",
    "location" : "Kollam",
    "employer_name" : "test test",
    "mobile" : "9123456987",
    "video" : "",
    "image" : "",
    "work_place" : "option1",
    "email" : "[email protected]",
    "skills" : [ 
        "php"
    ],
    "isActive" : true,
    "applied_users" : {
        "user_id" : ObjectId("58187e7551d244640626d7e1"),
        "accepted" : false
    },
    "subscribercol_docs" : []
}

here subscribercol_docs is empty array i need user info (name, address etc..),

Answer 1

Because there is no user_id field in local document its "applied_users.user_id" Try this

db.getCollection('employer_jobscol').aggregate([   {
      $unwind: "$applied_users"
   },
    {
      $lookup:
        {
          from: "subscribercol",
          localField: "applied_users.user_id", // <-- check here
          foreignField: "_id",
          as: "subscribercol_docs"
        }
   } 
])