Testing mongoDB (Mongoose) with Mocha Chai how do I retrieve from the mock database

I'm currently working on a rest api connecting to MongoDB

Here is my AuthController.js file as a user creates a profile the CardID is generated in the POST request. My question is how do I get the CardID that's been placed into the Database.

exports.register = [
// Validate Fields
body("firstName").isLength({ min: 1 }).trim().withMessage("First name must be specified.")
    .isAlphanumeric().withMessage("First name has non-alphanumeric characters."),
body("lastName").isLength({ min: 1 }).trim().withMessage("Last name must be specified.")
    .isAlphanumeric().withMessage("Last name has non-alphanumeric characters."),
body("email").isLength({ min: 1 }).trim().withMessage("Email must be specified.")
    .isEmail().withMessage("Email must be a valid email address.")
    .custom((value) => {
        return UserModel.findOne({email : value}).then((user) => {
            if (user) {
                return Promise.reject("E-mail already in use");
            }
        });
    }),
body("phoneNumber").isLength({ min: 1 }).trim().withMessage("Phone number must be specificed.")
    .isMobilePhone('en-GB').withMessage("Phone Number must be a valid UK number."),
body("pin").isLength({ min: 4, max: 4 }).trim().withMessage("Pin must be 4 characters.")
    .isInt().withMessage("Pin must be numeric only."),
    
// Check fields.
check("firstName").escape(),
check("lastName").escape(),
check("email").escape(),
check("phoneNumber").escape(),
check("pin").escape(),

(req, res) => {

    const errors = validationResult(req);

    if (!errors.isEmpty()) {
        // Display sanitized values/errors messages.
        return apiResponse.validationErrorWithData(res, "Validation Error.", errors.array());
    } else {

    try {
        bcrypt.hash(req.body.pin, 10, function(err, hash) {
            const user = new UserModel({
                UEID: "ueid-" + uuidv4(),
                firstName: req.body.firstName,
                lastName: req.body.lastName,
                email: req.body.email,
                phoneNumber: req.body.phoneNumber,
                pin: hash,
                cardID: utility.randomString(16, '#aA')
            })

            const newUser = user.save(function (err) {
                if (err) return apiResponse.ErrorResponse(res, err); 
                
                return apiResponse.successResponseWithData(res,"Registration Success.", newUser);
            });
        })
    } catch (err) {
        //throw error in json response with status 500.
        return apiResponse.ErrorResponse(res, err);
    }
}}   

]

Here is my test file as you can see I can't get to the CardID through the testData

 describe("Auth", () => {
      
    // Before each test we empty the database
    before((done) => { 
        UserModel.deleteMany({}, (err) => { enter code here
            done();           
        });        
    });

    // Prepare data for testing
    const testData = {
        "firstName": "test",
        "lastName": "testing",
        "email": "test@testing.com",
        "phoneNumber": "07899848930",
        "pin": "1234",
        "email": "test@testing.com"
    };

    /*
  * Test the /POST route
  */
    describe("/POST Tapcard", () => {

        it("It should ask for registration if cardID is not available", (done) => {
            chai.request(server)
                .post("/api/auth/tapcard/")
                .send({ "cardID": "dk3l4lkgmv3nt3f4" })
                .end((err, res) => {
                    res.should.have(401);
                    res.body.should.have.property("message").eql("Please register this card.");
                    done();
                });
        });

        it("It should login", (done) => {
            chai.request(server)
                .post("api/auth/tapcard/")
                .send({ "cardID" : 'GNmSZ7Zv7BYf4UuA' })
                .end((err, res) => {
                    res.should.have(200);
                    res.body.should.have.property("message").eql("Please enter pin.");
                    done();
                })
        })
    })

    /*
  * Test the /POST route
  */
    describe("/POST Login", () => {
        it("It should send validation error for Login", (done) => {
            chai.request(server)
                .post("/api/auth/tapcard/login")
                .send({"cardID": testData.cardID})
                .end((err, res) => {
                    res.should.have.status(400);
                    done();
                });
        });
    });