Atomic deep merge for document MongoDB - node.js

Question

I want to atomically deep merge a document in MongoDB. (I am using Node.js)
For example,
I have this document in my DB -
{
id: 123,
field1: {
field2: "abc"
}
}
And I am given this object:
{
field1: {
field3: "def"
}
}
So I'm looking for my document in the DB to change to this:
{
id: 123,
field1: {
field2: "abc",
field3: "def"
}
}
Of course I can do this in a non-atomic way by:
First fetching the document from the DB.
Then in JS do the merge and save it to a new object.
And finally overwrite the new item to the DB.
But I want this to happen in an atomic matter (so the read and the write stages will not happen separately). Is there any way of doing this?
I have heard the aggregation methods $merge and $mergeObjects might help me with that. Question is - are they atomic?
Thanks all :)

Answer 1

You can use the $function operator and go recursive.
Consider this doc in the DB:
{
field1: {
field6: "pre",
field2: "abc",
fieldX: {
aa: [1,2,3],
bb: "pre",
cc: "pre"
}
}
};
This is our candidate merge/overwrite doc:
var my_object = {
field1: {
field3: "def",
field6: "post",
fieldX: {
bb: "post",
zz: "post"
}
},
field4: 77,
field22: "qqq"
};
field4 and field22 are "easy"; they just get added to the doc. Where fields exist like field1, we have to test for objects vs. non-object. For objects, we will recursively descend into the db object and the candidate. For non-objects, we will let the candidate "win" and its value simply overwrites that in the db. This pipeline:
c = db.foo.aggregate([
{$replaceRoot: {newRoot: {$function: {
body: function(db_object, my_object) {
var process = function(myo, key, dbo) {
db_value = dbo[key];
if(undefined == db_value) {
// Easy; db does not have entry at all; just set it:
dbo[key] = myo[key];
} else {
// my_value has to be non-null because this function
// is driven from the keyset in myo.
my_value = myo[key];
if(db_value instanceof Object && my_value instanceof Object) {
// Both objects with same name; descend!
walkObj(db_value, my_value)
} else {
// my_object "wins" the slot and just overwrites dbo:
dbo[key] = myo[key];
}
}
};
var walkObj = function(dbo,myo) {
// Drive the walk from my_object; it overwrites/appends the db:
Object.keys(myo).forEach(function(k) {
process(myo, k, dbo);
});
}
walkObj(db_object, my_object); // entry point!
return db_object;
},
args: [ "$$CURRENT", my_object ],
lang: "js"
}}
}}
// Take each doc and put it back using _id as the key:
,{$merge: {
into: "foo",
on: [ "_id" ],
whenMatched: "merge",
whenNotMatched: "fail"
}}
]);
will produce this result in the DB:
{
_id: ObjectId("63f0f5e45d0b16d1033771e7"),
field1: {
field6: 'post',
field2: 'abc',
fieldX: { aa: [ 1, 2, 3 ], bb: 'post', cc: 'pre', zz: 'post' },
field3: 'def'
},
field22: 'qqq',
field4: 77
}