How do I partially update an object in MongoDB so the new object will overlay / merge with the existing one

Question

Given this document saved in MongoDB

{ 

_id : ..., 

some_key: 

{ 

param1 : "val1", 

param2 : "val2",

param3 : "val3" 

} 

}

An object with new information on param2 and param3 from the outside world needs to be saved

var new_info = { 

param2 : "val2_new",

param3 : "val3_new" 

};

I want to merge / overlay the new fields over the existing state of the object so that param1 doesn't get removed

Doing this

db.collection.update({ _id:...},{ $set:{ some_key : new_info }}

Will lead to MongoDB is doing exactly as it was asked, and sets some_key to that value. replacing the old one.

{ 

_id : ...,

some_key: {

 param2 : "val2_new", 

 param3 : "val3_new" 

} 

}

What is the way to have MongoDB update only new fields (without stating them one by one explicitly)? to get this:

{

_id : ..., 

some_key: { 

param1 : "val1", 

param2 : "val2_new", 

param3 : "val3_new" 

} 

}

I'm using the Java client, but an example will be appreciated

Answer 1

To partially update an object in MongoDB you can do something like as follows:-:

db.collection.update( { _id:...},{ $set: someObjectWithNewData }

Answer 2

You first have to query the document, figure out what you want to $set and then update it (using the old values as a matching filter to make sure you don't get concurrent updates in between).