CODEWITHSUNDEEP
mongodbmongodb-query
Mano
Mano

Reputation: 55

Can I use the existing document field value in the $push of mongodb ?, I tried the below approach , its not working

db.getCollection('test').update({
  "b_id": "3"
},
{
      "$push": {"books": {
        "status": "available",
        "shelf_id": "$shelf_id",
        "rack_no": "$rack_no",
        "book_id": new ObjectId()
      }
  }
})

shelf_id and rack_no are already available in the document. Here, I need to create the array "books" which includes status, shelf_id, rack_no and book_id.

I am not able to get the existing value of shelf_id and rack_no in this case. It considers $rack_id as string instead the value from the document. Please suggest a way.

Upvotes: 0

Views: 58

Answers (1)

Joe
Joe

Reputation: 28366

Yes, you can do that in MongoDB 4.2, using the pipeline form of update, but not with $push.

Passing an array of pipeline stages as the second argument to update will allow you to use aggregation operators.

The $push aggregation operator behaves very differently from the $push update operator, so in a pipeline use $set with $concatArrays, like:

db.getCollection('test').update({
  "b_id": "3"
},
[{$set:
    {
      books: {$concatArrays:[
                "$books",
                [{
                  "status": "available",
                  "shelf_id": "$shelf_id",
                  "rack_no": "$rack_no",
                  "book_id": new ObjectId()
                }]
      ]}
    }
}])

Upvotes: 1

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