Set field of all documents with given value only if not existing in Mongo

Use update method with the multi flag set to true

  { "welcomeAck": { "$exists": false } },
  { "$set": { "welcomeAck": true } },
  { "multi": true }

Or the equivalent shortcut with updateMany:

  { "welcomeAck": { "$exists": false } },
  { "$set": { "welcomeAck": true } }

Reference -

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Adrian Matei

Adrian Matei
Life force expressing itself as a coding capable human being

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