CODEWITHSUNDEEP
mongodb-queryaggregation-frameworkspring-mongodbspring-mongo
Md Enayat
Md Enayat

Reputation: 167

Join two collection in mongodb

I'm new in mongodb. Could you please tell me how to perform join operation in this. I've two collection:

Collection 1 ("user")

{
 _id: "d04d53dc-fb88-433e-a1c5-dd41a68d7655",
 userName: "XYZ User",
 age: 12
}

Collection 2 ("square")

{
 _id: "ef6f6ac2-a08a-4f68-a63c-0b4a70285427",
 userId: "d04d53dc-fb88-433e-a1c5-dd41a68d7655",
 side: 4,
 area: 16
}

Now I want to retrieve the data from collection 2 is like this. Expected output:

{
 _id: "ef6f6ac2-a08a-4f68-a63c-0b4a70285427",
 userId: "d04d53dc-fb88-433e-a1c5-dd41a68d7655",
 userName: "XYZ User",
 side: 4,
 area: 16
}

Thanks in advance :)

Upvotes: 3

Views: 3747

Answers (2)

Adarsh Kumar
Adarsh Kumar

Reputation: 21

You can keep the first documentid (_id) in the second document as userId for refrence and after that, you can use the join feature supported by MongoDB 3.2 and later versions. You can use joins by using an aggregate query. You can do it using the below example :

    db.user.aggregate([

    // Join with square table
    {
        $lookup:{
            from: "square",       // other table name
            localField: "_id",   // name of user table field
            foreignField: "userId", // name of square table field
            as: "square"         // alias for userinfo table
        }
    },
    {   $unwind:"$user_info" },     // $unwind used for getting data in object or for one record only

     
    // define some conditions here 
    {
        $match:{
            $and:[{"userName" : "XYZ User"}]
        }
    },

    // define which fields are you want to fetch
    {   
        $project:{
            _id: 1,
            userId: "$square.userId",
            userName: 1,
            side: "$square.side",
            area: "$square.area"
        } 
    }
]);

The Result will be

     {
      _id: "ef6f6ac2-a08a-4f68-a63c-0b4a70285427",
      userId: "d04d53dc-fb88-433e-a1c5-dd41a68d7655",
      userName: "XYZ User",
      side: 4,
      area: 16
     }

Cheers

Upvotes: 2

rickhg12hs
rickhg12hs

Reputation: 11912

Here's one way to do it.

db.square.aggregate([
  {
    "$lookup": {
      "from": "user",
      "localField": "userId",
      "foreignField": "_id",
      "as": "userDoc"
    }
  },
  {
    "$set": {
      "userName": {
        "$first": "$userDoc.userName"
      }
    }
  },
  { "$unset": "userDoc" }
])

Try it on mongoplayground.net.

Upvotes: 4

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