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

Use update method with the multi flag set to true

db.users.update(
  { "welcomeAck": { "$exists": false } },
  { "$set": { "welcomeAck": true } },
  { "multi": true }
);

Or the equivalent shortcut with updateMany:

db.users.updateMany(
  { "welcomeAck": { "$exists": false } },
  { "$set": { "welcomeAck": true } }
);

Reference - https://docs.mongodb.com/manual/tutorial/update-documents/

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