CODEWITHSUNDEEP
mongodbmongooseaggregation-framework
Michael Mueller
Michael Mueller

Reputation: 311

Set value of string to first element of array

For a given document, I am attempting to set a field (string) as the first element of an array of strings. Consider a collection myPlaces with the following document:

{
    "name" : "Place Q",
    "images" : [
        "foo-1",
        "foo-2",
        "foo-3"
    ]
}

I am looking to create the following:

{
    "name" : "Place Q",
    "images" : [
        "foo-1",
        "foo-2",
        "foo-3"
    ],
    "image" : "foo-1"
}

So in the mongo shell, I execute:

db.myPlaces.update(
   {
      name: "Place Q"
   },
   {
      $set:
         {
            image: images.0
         }
   }
)

This throws the error SyntaxError: missing } after property list which I don't understand. If I enclose the array in quotes "images.0" the value of image is set as "images.0", not "foo-1".

I've been reviewing the Mongodb documentation and see a lot of different examples but not something that makes clear to me what is probably a simple syntax for accessing an array value.

Upvotes: 0

Views: 1034

Answers (3)

Hardik Shah
Hardik Shah

Reputation: 4200

Here what you can do is:

  • Find the document first.
  • Iterate on that (You must have only one value, but it will work for multiple documents as well)
  • Update document.

Note:- Efficient for the small records. Bulk record will have lots of traffic on DB.

===== Will work for all version of MongoDB ======

Data Before:

{
        "_id" : ObjectId("5d53fda5f5fc3d6486b42fec"),
        "name" : "Place Q",
        "images" : [ 
            "foo-1", 
            "foo-2", 
            "foo-3"
        ]
    }

Query:

db.collection.find({ name: "Place Q" }).forEach(function(document) {
    db.collection.update(
        {}, 
        { "$set": { "image": document.images[0] } }
    );
})

Data After

{
        "_id" : ObjectId("5d53fda5f5fc3d6486b42fec"),
        "name" : "Place Q",
        "images" : [ 
            "foo-1", 
            "foo-2", 
            "foo-3"
        ],
        "image" : "foo-1"
    }

Upvotes: 0

Max CodeSmith
Max CodeSmith

Reputation: 442

Can you try this please :

db.myPlaces.update(
    {
        name: "Place Q"
    },{
        $set:
            {
                image: db.myPlaces.find( { "images.0" } )
            }
    }
)

Upvotes: 0

Praveen Rewar
Praveen Rewar

Reputation: 961

The following query works on MongoDB 4.2, It might not work on previous versions.

db.myPlaces.update(
  {
    name: "Place Q"
  },
  [
    {
      $set: {
        image: {
          $arrayElemAt: ["$images",0]
        }
      }
    }
  ]
)

It's also very efficient as it is using $set pipeline provided in MongoDB 4.2+ versions.

Upvotes: 4

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