How can I modify the resulting query in Mongoose without affecting the actual document?

Status (as of 5/6/2020): Solved, see the identified answer below.

I hope all is well despite the global crisis we are experiencing right now. I am currently doing a school web project and need to render a specific feature assigned to me. I am using Mongoose with Express, and Handlebars for templating. Please see the attached model schema and explanation below.

collegeModel - Collection A

var collegeSchema = new Schema({
	shortName: {type: String},	//value that I intend to synchronously query its occurrence with Collection B
	longName: {type: String},
	logo: {type: String},
	contactUs:{
		telNum: {type: String},
		faxNum: {type: String},
		email: {type: String}
	},
	aboutUs: {type: Array},
	visionMission: {type: String},
	coreValues: {type: String},
	goals: {type: String},
	founderBio: {type: String},
	philosophy: {type: String},
	icon: {type: String}
});

professorModel - Collection B

var professorSchema = new Schema({
	profNumber: {type: Int32},
	college: {type: String},	//value to be compared with shortName
	gender: {type: String},
	profName: {type: String},
	profCourse: {type: String}
});

Pseudocode - Desired logic to be achieved

app.get('/testCount', function(req,res) {
	collegeModel.find({}).lean().exec(function(err,collegeRes){
		var collegeObject = [];
		collegeRes.forEach(function(document){
			professorModel.countDocuments({college:document.shortName}, function(err2,professorCount){
				document.count = professorCount;
				collegeObject.push(document);	//doing a console.log(collegeObject) would return empty objects [].
			});
		});
	});
});

I don't know what I'm doing wrong and I know document.count exists since it returns a value everytime I do console.log(document.count) but when it's pushed it becomes []. Hope you can help me achieve my goal. Thanks!

Your queries resolve asynchronously, you have to find a way to wait for all of them to complete to make sure you have all the data you need.

One way to solve this is using async/await (Node.js >= 7.6.0)

app.get('/testCount', async function(req, res) { // note the async keyword
  const collegeRes = await collegeModel.find({}).lean().exec() // .exec() returns a Promise, so you can `await` it.
  const resultPromises = collegeRes.map(async college => { // arrow function is equivalent to function in this context
    const professorCount = await professorModel.countDocuments({ college: college.shortName })
    college.count = professorCount
    return college
  })
  const collegeObject = await Promise.all(resultPromises)
  console.log(collegeObject)
})

A bit more readable would be using Promise.map from bluebird or you can also use other promise utility library

  const collegeObject = await Promise.map(collegeRes, college => {
    const professorCount = await professorModel.countDocuments({ college: college.shortName })
    college.count = professorCount
    return college
  })
  console.log(collegeObject)