Question

Suppose I have a doc structure like this:

thing: {
  name: {
    first: "John",
    last: "Doe"
  }
}

say I want to update the last name only. Which command do I send to update?

$set: {
  name: {
    first: "Connor"
  }
}

or

$set: {
  "name.first": "Connor"
}

Is there a difference? Or a preference? I much prefer the first since it resembles the actual document, but mongodb documentation uses the second method.

Answer 1

$set command will take the key and overwrite whatever was stored in it by the value you pass. So in this case

$set: {
  name: {
    first: "Connor"
  }
}

the whole subdocument name with potentially rich structure is getting replaced with a simple {first: 'Connor'}.

Similar thing is happening in the second case, only it is one level deeper. In this case it’s a string, but it could be a hash as well.

$set: {
  "name.first": "Connor"
}

You can update fields at arbitrary depth level by constructing proper dotted name. Here’s a slightly contrived example

db.collection.update(query, {$set: {'stats.daily.20120622.mainpage.visited': 1}});