How to update partial document in single command?

Question

I have a document like this:

{
   _id : ...,
   key : { 
        a : 1,
        b : 2,
        c : 3
   }
}

And I will update this collection with this:

var data = {
    b : 4,
    c : 5
};

And I want my document be update to this:

{
   _id : ...,
   xxx : { 
        a : 1,
        b : 4,
        c : 5
   }
}

So if I do this:

db.myColl.update(  _id , { $set: {"xxx": data } );

it will end up a document like this:

{
   _id : ...,
   xxx: { 
      b : 4,
      c : 5
   }
}

So I have to do it twice:

db.myColl.update(  _id , { $set: {"xxx.b": data.b } );
db.myColl.update(  _id , { $set: {"xxx.c": data.c } );

Is it possible to do it in single command?

No, thats the only way I have seen. $set replaces any sub properties with the new values, so if its missing one, it considers that deleted. — Rob
– Rob, Commented May 20, 2015 at 23:49

JohnnyHK · Accepted Answer · 2015-05-21 02:09:30Z

1

You can do this by combining both of the $set terms into a single update:

db.myColl.update(_id, { $set: {"xxx.b": data.b, "xxx.c": data.c } });

answered May 21, 2015 at 2:09

JohnnyHK

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1 Comment

Sylvain Leroux Over a year ago

... I would even say this should be done that way -- so the update is atomic. When the OP uses two separate updates, there is a possibility for concurrent reads on the DB to see a a partially updated document.

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