How is findById() + save() different from update() in MongoDB - mongodb

Question

While trying to update a MongoDB document using Mongoose, can I use a findById() with a save() in the callback, or should I stick with traditional update methods such as findByIdAndModify, findOneAndModify, update(), etc.? Say I want to update the name field of the following document (please see a more elaborate example in the edit at the end, which motivated my question):
{
"_id": ObjectId("123"),
"name": "Development"
}
(Mongoose model name for the collection is Category)
I could do this:
Category.update({ "_id" : "123" }, { "name" : "Software Development" }, { new: true })
or I could do this:
Category.findById("123", function(err, category) {
if (err) throw err;
category.name = "Software Development";
category.save();
});
For more elaborate examples, it feels easier to manipulate a JavaScript object that can simply be saved, as opposed to devising a relatively complex update document for the .update() operation. Am I missing something fundamentally important?
Edited 7/21/2016 Responding to the comment from #Cameron, I think a better example is warranted:
{
"_id": ObjectId("123"),
"roles": [{
"roleId": ObjectId("1234"),
"name": "Leader"
}, {
"roleId": ObjectId("1235"),
"name": "Moderator"
}, {
"roleId": ObjectId("1236"),
"name": "Arbitrator"
}]
}
What I am trying to do is remove some roles as well as add some roles in the roles array of sub-documents in a single operation. To add role sub-documents, $push can be used and to remove role sub-documents, $pull is used. But if I did something like this:
Person.update({
"_id": "123"
}, {
$pull : {
"roles" : {
"roleId" : {
$in : [ "1235", "1236" ]
}
}
},
$push : {
"roles" : {
$each: [{
"roleId" : ObjectId("1237"),
"name" : "Developer"
}]
}
}
}
When I try to execute this, I get the error Cannot update 'roles' and 'roles' at the same time, of course. That's when I felt it is easier to find a document, manipulate it any way I want and then save it. In that scenario, I don't know if there is really any other choice for updating the document.

Answer 1

I typically like to use findById() when I am performing more elaborate updates and don't think you are missing anything fundamentally important.
However one method to be aware of in mongoose is findByIdAndUpdate(), this issues a mongodb findAndModify update command and would allow you to perform your first example with the following code: Category.findByIdAndUpdate("123", function(err, savedDoc) {...}).