Question

I can’t see the difference between using it:

update({"name" : "nick"}, {"$set" : {"age" : 50}})

and not using it;

update({"name" : "nick"}, {"age" : 50})

from the example in documentation. It’s not clear to me.

Thanks for the comment but if I use {"$set" : {"array_field" : [{'f' : 'v'}] }} it adds {'f' : 'v'} to the array instead of replacing the array with [{'f' : 'v'}], so why doesn’t $set replace the array with the new one?

Answer 1

update({"name" : "nick"}, {"age" : 50}) replaces the complete object with only {"age" : 50}.

Running find({"name" : "nick" }) after running the above update would return no document found error.

Using $set would allow to update only the “age” param of the original object.

Karl Seguin’s Little Mongodb Book has a very good explanation about the difference between replace and set.