Overwrite value and create key while update query in mongodb

Question

I have a mongodb collection that looks like this:

{
     "_id" : ObjectId("60471bd482c0da3c0e70d26f"),
     "owner" : "John",
     "propAvailable" : {
        "val1" : true
    }
},
{
    "_id" : ObjectId("60471bd482c0da3c0e76523f"),
    "owner" : "Matt",
    "propAvailable" : {
    "val1" : {
       "val2" : true  
    }
}

I need to run an update query on this collection that will update the value of the 'propAvailable' key such that

db.collection('props').update({'owner' : 'John'} , {$set : {'propAvailable.val1.val2' : true}});

This query works if the document already looks like the second one but gives the error: Cannot create field 'val2' in element {'val1': true} if the document format is the first one. Is there a way to write this query so that it overwrites the boolean 'true' and replaces it with the object {'val2' : true}

Answer 1

You can use:

db.collection.update({
  "owner": "John"
},
{
  $set: {
    "propAvailable.val1": {
      val2: true
    }
  }
})

To create val2: true inside propAvailable.val1 and replace its current content.

As you can see working on the playground

Answer 2

If you're using Mongo version 4.2+ you can use pipelined updates to achieve this, like so:

db.collection.updateMany({
  owner: "John"
},
[
  {
    $set: {
      "propAvailable.val1": {
        $mergeObjects: [
          {
            $cond: [
              {
                $eq: [
                  "object",
                  {
                    $type: "$propAvailable.val1"
                  }
                ]
              },
              "$propAvailable.val1",
              {}
            ]
          },
          {
            val2: true
          }
        ]
      }
    }
  },
])

Mongo Playground

For older mongo versions this is impossible to do in 1 query if objects potentially have additional fields under val1 you want to preserve. You will have to either read and update, or execute two different updates for each case.