Renaming Field Within an Array in MongoDB

Question

I need to update the name of field in a collection. The problem is that the field in question is within an array. So I'm trying to determine the correct way do this. I tried this to accomplish renaming the field that exists within the "plans" array: :

db.customers.updateMany( {}, { $rename: { "plans" : { "subscriptionType": "membershipType" } } } );

But this won't work. What's the correct way to handle this kind of transformation of a field within an array?

The data looks like this:

{
  _id: 123,
  prop1: value,
  prop2: value,
  prop3: value,
  plans: [
    subscriptionType: value,
    otherProp: value,
    otherProp: value
  ]
}

The answers in the other question are very old. Wondering if there is new functionality for Mongo to handle this kind of transformation now? — Muirik
– Muirik, Commented Aug 21, 2018 at 17:54

mickl · Accepted Answer · 2018-08-21 18:13:44Z

5

You can use Aggregation Framework's $addFields to override plans field and $map operator to rename field inside an array. Then you can use $out to override existing collection:

db.customers.aggregate([
    {
        $addFields: {
            plans: {
                $map:{
                    input: "$plans",
                    as: "plan",
                    in: {
                        membershipType: "$$plan.subscriptionType",
                        otherField: "$$plan.otherField",
                        otherField2: "$$plan.otherField2"
                    }
                }
            }
        }
    },
    {
        $out: "customers"
    }
])

Alternatively you can do that dynamically. In this solution you don't have to explicitly specify other field names:

db.customers.aggregate([
    {
        $addFields: {
            plans: {
                $map:{
                    input: "$plans",
                    as: "plan",
                    in: {
                        $mergeObjects: [
                            { membershipType: "$$plan.subscriptionType" },
                            { 
                                $arrayToObject: {
                                    $filter: {
                                        input: { $objectToArray: "$$plan" },
                                        as: "plan",
                                        cond: { $ne: [ "$$plan.k", "subscriptionType" ] }
                                    }
                                } 
                            }
                        ]
                    }
                }
            }
        }
    },
    {
        $out: "customers"
    }
])

Using $objectToArray to $filter out old key-value pair and the using $mergeObjects to combine that filtered object with new renamed field.

edited Aug 21, 2018 at 18:13

answered Aug 21, 2018 at 17:55

mickl

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6 Comments

Muirik Over a year ago

I added more detail to my data structure. Is there a way to do this without overriding other fields within the plans array? Sorry, should have been more specific to begin with.

mickl Over a year ago

@Muirik that's why I wanted to see the structure. So the simplest way would be to specify them as well like in my edited answer. Let met know if that's okay for you. Otherwise we can try some tricks with $mergeObjects :)

mickl Over a year ago

Anyway, I've added the "dynamic" approach

Patrik Šimunič Over a year ago

@mickl Works amazingly! You saved me potential hours of work :D thank you!!!

Alikhan Oitan Over a year ago

Sorry if I am late @mickl, but why do we need $$plan.k in $ne? is that some field name or what? trying to implement your second solution. Thanks in advance!

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