Copy first array value to another field in MongoDB

Question

I have an old list of products that store the descriptions in an array at index [0]. The model is set up as a string. So my plan is to extract that value and add it to a temporary field. Step 2 would be to take that value and copy it to the original field. This is the 'wrong' product I want to fix.

{_id : 1 , pDescription : ['great product']},
{_id : 2 , pDescription : ['another product']}

All I want to is to change the array to a string like this:

{_id : 1 , pDescription : 'great product'},
{_id : 2 , pDescription : 'another product'}

I have tried this to create the temporary description:

    Products.aggregate([
    {
        $match: {
            pDescription: {
                $type: "array"
            }
        }
        },
    {
        $set: {
            pDescTemp: {
                $first: "$pDescription"
            }
        }
            }
        ]).exec((err, r) => {
       // do stuff here
   });

The command works fine without the $first command. The error reads: MongoError: Unrecognized expression '$first'

Any tips on how to fix this are appreciated! Thanks!

codemonkey · Accepted Answer · 2021-01-29 01:17:34Z

2

I believe this is what you need to update your pDescription field to be equal to the first element of the array already stored as pDescription:

db.Products.updateMany({},
[
  {
    $set: {
      pDescription: {
        $arrayElemAt: [
          "$pDescription",
          0
        ]
      }
    }
  }
])

edited Jan 29, 2021 at 1:17

answered Jan 29, 2021 at 0:56

codemonkey

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5 Comments

Caldera500 Over a year ago

Fantastic. Thank you so much codemonkey!!

codemonkey Over a year ago

@Caldera500 No problem. Just remember that aggregation in and of itself does not update anything, it's just a tool to accumulate and display data, so Products.aggregate is not the way to go. Rather you need an aggregation pipeline inside an update operator.

Caldera500 Over a year ago

This makes totally sense. During my research I found out that you can combine aggregation with updates. I guess this is mostly used for other applications. The update call you are describing is much better for my use case. Also extremely elegant and fast. I updated 5k document in less than 0.5 secs. The Mongo docs usually only show the most basic use cases. I guess I still have much to learn here. Thanks again!

Konrad Viltersten Over a year ago

How would it need to be altered to store the first element not onto the source itself but to another field? I tried the above but got The dollar ($) prefixed field '$arrayElemAt' in 'plonk.$arrayElemAt' is not valid for storage. Here, plonks is my array to copy the first element from and plonk is the field to store it in.

codemonkey Over a year ago

@KonradViltersten If I understand you correctly, here is what you need: mongoplayground.net/p/iiEkmA8ecMU

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