MongoDB - How to join two documents using $lookup

Question

I have been trying to join two collections in MongoDB using the aggregate function but it seems it's not working for me, When I run API using lookup it shows me an empty collection [],

I have tried the following:

db.student.aggregate([
  {
    "$match": {
      _id: "63c4c245188267e988d690e2"
    },
    
  },
  {
    "$lookup": {
      "from": "wall",
      "localField": "_user",
      "foreignField": "_id",
      "as": "wallpost"
    }
  }
])

Result: Here is the result I m getting after lookup:

[
  {
    "_id": "63c4c245188267e988d690e2",
    "hereFor": [
      {
        "mainTag": "study",
        "subtag": [
          "studybuddy",
          "findtutor"
        ]
      }
    ],
    "lastName": "Kumar",
    "name": "Kamal",
    "profilePicture": [
      "https://airustudentimages.s3.ca-central-1.amazonaws.com/1673588594404-ba7777ef676f439f86aa612e8be67fd9"
    ],
    "wallpost": []
  }
]

Collections Collection I'm using in the query.

Student

student: [
    {
      "_id": "63c4c245188267e988d690e2",
      "name": "Kamal",
      "lastName": "Kumar",
      "profilePicture": [
        "https://airustudentimages.s3.ca-central-1.amazonaws.com/1673588594404-ba7777ef676f439f86aa612e8be67fd9"
      ],
      "hereFor": [
        {
          "mainTag": "study",
          "subtag": [
            "studybuddy",
            "findtutor"
          ]
        }
      ],
      
    },
    {
      "_id": "63c3965c201a1d738ab6867e",
      "name": "Suraksha",
      "lastName": "Singh",
      "profilePicture": [
        "https://airustudentimages.s3.ca-central-1.amazonaws.com/1673762449670-a8bdf9b9b0faf3ad84e0a5bc76e32fb8"
      ],
      "hereFor": [
        {
          "mainTag": "study",
          "subtag": [
            "studybuddy",
            "findtutor"
          ]
        }
      ],
      
    }
  ],

Wall

"wall": [
    {
      "_id": "63c4c92a188267e988d690e3",
      "_user": "63c3965c201a1d738ab6867e",
      "isSponsered": false,
      "description": "Hello test case ",
      "tag": {
        "mainTag": "hostels"
      },
      "createdAt": "1673766717308",
      "updatedAt": "1673766717308",
      
    },
    {
      "_id": "63c4cc2b188267e988d690e5",
      "_user": "63c3965c201a1d738ab6867e",
      "isSponsered": false,
      "description": "Hello test case 22 ",
      "tag": {
        "mainTag": "hostels"
      },
      "createdAt": "1673766717308",
      "updatedAt": "1673766717308",
      
    },
    
  ],

Have a look at https://mongoplayground.net/p/2moDXi3lygL

Fourchette · Accepted Answer · 2023-01-17 08:39:17Z

1

Your query is on Student collection. So the localField should be _id and and the foreignField should be _user (from wall collection).

Then it works fine.

db.student.aggregate([
  {
    "$match": {
      _id: "63c3965c201a1d738ab6867e"
    },
    
  },
  {
    "$lookup": {
      "from": "wall",
      "localField": "_id",
      "foreignField": "_user",
      "as": "wallpost"
    }
  }
])

https://mongoplayground.net/p/r1JeQIlM7AA

answered Jan 17, 2023 at 8:39

Fourchette

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Comments

Yong Shun · Accepted Answer · 2023-01-17 08:37:45Z

1

You mix up the localField and foreignField.

From Equality Match with a Single Join Condition:

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

It should be:

{
  "$lookup": {
    "from": "wall",
    "localField": "_id",
    "foreignField": "_user",
    "as": "wallpost"
  }
}

Demo @ Mongo Playgound

answered Jan 17, 2023 at 8:37

Yong Shun

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