How to query and update mongodb subdocuments

Question

So, I am trying to update a subdocument in MongoDB, I am using express and I have the below route for finding the exact object I want to update, at the moment I can only replace the whole document with a new one, is it ok if I leave it like this or is there a MongoDB/mongoose operation/function that can help me to just update the fields based on what the user provides, for example, if they just provide a title to be updated or a priority. I know that I have to not use the schema to update just one property, I am trying to find an alternative to using a chain fo if - elseif or switch statements.

app.put('/updateTodoInProject/:id/:todo_id', (req,res)=>{ //':id' represents the id of the project in which the todo is
    const collection = db.collection('projects');
    const todo = new Todo({title: req.body.title, description: req.body.description, priority: req.body.priority});
    collection.updateOne({_id: mongo.ObjectID(req.params.id),
        todos: {$elemMatch: {_id: mongo.ObjectID(req.params.todo_id)}}
    },{
        $set: {
            "todos.$": todo
        }
    },function(err, results) {
        if (err){
            console.log(err);
            res.send('');
            return
        }
        res.send(results)
    });
});

The object I am trying to update looks something like this:

{
 _id: "5e3bf72db5074c1e205409f5",
 todos: [
  {
    _id: "5e42bef746bae7728c68384f",
    title: "cool title 1",
    description: "cool description 2",
    priority: 1
  }
 ],
 title: "hjskadjshd"
}

Answer 1

You can create a complete update object outside based on the details entered by user & then update values in your query like this

app.put('/updateTodoInProject/:id/:todo_id', (req,res)=>{ //':id' represents the id of the project in which the todo is
    const collection = db.collection('projects');
    var updateObj = {'todos.$.title': req.body.title, 'todos.$.description': req.body.description, 'todos.$.priority': req.body.priority};
    collection.updateOne({_id: mongo.ObjectID(req.params.id),
        todos: {$elemMatch: {_id: mongo.ObjectID(req.params.todo_id)}}
    },{
        $set: updateObj
    },function(err, results) {
        if (err){
            console.log(err);
            res.send('');
            return
        }
        res.send(results)
    });
});