MongoDB - Mongoose - TypeError: save is not a func

I am attempting to perform an update to a MongoDB document (using mongoose) by first using .findById to get the document, then updating the fields in that document with new values. I am still a bit new to this so I used a tutorial to figure out how to get it working, then I have been updating my code for my needs. Here is the tutorial: MEAN App Tutorial with Angular 4. The original code had a schema defined, but my requirement is for a generic MongoDB interface that will simply take whatever payload is sent to it and send it along to MongoDB. The original tutorial had something like this:

exports.updateTodo = async function(todo){
    var id = todo.id

    try{
        //Find the old Todo Object by the Id

        var oldTodo = await ToDo.findById(id);
    }catch(e){
        throw Error("Error occured while Finding the Todo")
    }

    // If no old Todo Object exists return false
    if(!oldTodo){
        return false;
    }

    console.log(oldTodo)

    //Edit the Todo Object
    oldTodo.title = todo.title
    oldTodo.description = todo.description
    oldTodo.status = todo.status


    console.log(oldTodo)

    try{
        var savedTodo = await oldTodo.save()
        return savedTodo;
    }catch(e){
        throw Error("And Error occured while updating the Todo");
    }
}

However, since I don't want a schema and want to allow anything through, I don't want to assign static values to specific field names like, title, description, status, etc. So, I came up with this:

exports.updateData = async function(update){
    var id = update.id

    // Check the existence of the query parameters, If they don't exist then assign a default value
    var dbName = update.dbName ? update.dbName : 'test'
    var collection = update.collection ? update.collection : 'testing'; 

    const Test = mongoose.model(dbName, TestSchema, collection);

    try{
        //Find the existing Test object by the Id
        var existingData = await Test.findById(id);
    }catch(e){
        throw Error("Error occurred while finding the Test document - " + e)
    }

    // If no existing Test object exists return false
    if(!existingData){
        return false;
    }

    console.log("Existing document is " + existingData)

    //Edit the Test object
    existingData = JSON.parse(JSON.stringify(update))

    //This was another way to overwrite existing field values, but
    //performs a "shallow copy" so it's not desireable
    //existingData = Object.assign({}, existingData, update)

    //existingData.title = update.title
    //existingData.description = update.description
    //existingData.status = update.status

    console.log("New data is " + existingData)

    try{
        var savedOutput = await existingData.save()
        return savedOutput;
    }catch(e){
        throw Error("An error occurred while updating the Test document - " + e);
    }
}

My original problem with this was that I had a lot of issues getting the new values to overwrite the old ones. Now that that's been solved, I am getting the error of "TypeError: existingData.save is not a function". I am thinking the data type changed or something, and now it is not being accepted. When I uncomment the static values that were in the old tutorial code, it works. This is further supported by my console logging before and after I join the objects, because the first one prints the actual data and the second one prints [object Object]. However, I can't seem to figure out what it's expecting. Any help would be greatly appreciated.

EDIT: I figured it out. Apparently Mongoose has its own data type of "Model" which gets changed if you do anything crazy to the underlying data by using things like JSON.stringify. I used Object.prototype.constructor to figure out the actual object type like so:

console.log("THIS IS BEFORE: " + existingData.constructor);
existingData = JSON.parse(JSON.stringify(update));
console.log("THIS IS AFTER: " + existingData.constructor);

And I got this:

THIS IS BEFORE: function model(doc, fields, skipId) {
  model.hooks.execPreSync('createModel', doc);
  if (!(this instanceof model)) {
    return new model(doc, fields, skipId);
  }
  Model.call(this, doc, fields, skipId);
}
THIS IS AFTER: function Object() { [native code] }

Which showed me what was actually going on. I added this to fix it:

existingData = new Test(JSON.parse(JSON.stringify(update)));

On a related note, I should probably just use the native MongoDB driver at this point, but it's working, so I'll just put it on my to do list for now.

You've now found a solution but I would suggest using the MongoDB driver which would make your code look something along the lines of this and would make the origional issue disappear:

// MongoDB Settings
const MongoClient = require(`mongodb`).MongoClient;
const mongodb_uri = `mongodb+srv://${REPLACE_mongodb_username}:${REPLACE_mongodb_password}@url-here.gcp.mongodb.net/test`;
const db_name = `test`;
let db; // allows us to reuse the database connection once it is opened

// Open MongoDB Connection
const open_database_connection = async () => {
  try {
    client = await MongoClient.connect(mongodb_uri);
  } catch (err) { throw new Error(err); }
  db = client.db(db_name);
};


exports.updateData = async update => {

  // open database connection if it isn't already open
  try {
    if (!db) await open_database_connection();
  } catch (err) { throw new Error(err); }


  // update document
  let savedOutput;
  try {
    savedOutput = await db.collection(`testing`).updateOne( // .save() is being depreciated
      { // filter
        _id: update.id // the '_id' might need to be 'id' depending on how you have set your collection up, usually it is '_id'
      },
      $set: { // I've assumed that you are overwriting the fields you are updating hence the '$set' operator
        update // update here - this is assuming that the update object only contains fields that should be updated
      }

      // If you want to add a new document if the id isn't  found add the below line
      // ,{ upsert: true }

    );
  } catch (err) { throw new Error(`An error occurred while updating the Test document - ${err}`); }


  if (savedOutput.matchedCount !== 1) return false; // if you add in '{ upsert: true }' above, then remove this line as it will create a new document

  return savedOutput;
}

The collection testing would need to be created before this code but this is only a one-time thing and is very easy - if you are using MongoDB Atlas then you can use MongoDB Compass / go in your online admin to create the collection without a single line of code...

As far as I can see you should need to duplicate the update object. The above reduces the database calls from 2 to one and allows you to reuse the database connection, potentially anywhere else in the application which would help to speed things up. Also don't store your MongoDB credentials directly in the code.