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MongoDB - Mongoose - TypeError: save is not a function

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.

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