In mongodb how do I replace array field with null value to an empty array

Question

I want to replace an array field in document that has value of null to an empty array []. I tried this query but it's not working. Please help.

Basically I want testArray: null to be testArray: []

db.myTestCollection.aggregate( { "_id": ObjectId('1120191011212112') },
   [ { $set: { testArray: { $ifNull: [ { $concatArrays: [ "$testArray", [] ] } ] } } }
])

user20042973 · Accepted Answer · 2022-10-04 16:57:15Z

2

Looks like you mostly have a syntax error. But you can also move the $ifNull logic into the filter predicate as well so that no update is considered/applied if the existing testArray field isn't already null. So:

db.collection.update({
  _id: 1,
  "testArray": null
},
[
  {
    $set: {
      testArray: []
    }
  }
])

Working playground example here.

answered Oct 4, 2022 at 16:57

user20042973

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