Jest mock mongoose.startSession() throws error - mongodb

Question

i'm implemented transaction in the post method. it was work fine. But now I have to update unit test case for that method. I tried to mock startSession() and startTransaction() to check toHaveBeenCalled.But while running test case i got like MongooseError: Connection 0 was disconnected when calling startSession``. I am new to that so i don't know how to mock that?.
Method:
static post = (funcCall: Promise<Document>) => async (response: Response, fields?: string[]) => {
const session = await startSession();
session.startTransaction();
try {
const dbResponse = await funcCall; // model.save(request.body)
// commit the changes if everything was successful
await session.commitTransaction();
success(pick(dbResponse, fields ? fields : ['_id']), 201)(response);
} catch (error) {
// this will rollback any changes made in the database
await session.abortTransaction();
throwError(error);
} finally {
// ending the session
session.endSession();
}
};
My Test case:
it('should perform post when valid parameters is passed.', async () => {
// Preparing
const mockSaveReturn = {
_id: objectID,
test_name: 'sample',
};
jest.spyOn(mongoose, 'startSession')
const spySave = jest.spyOn(modelPrototype.prototype, 'save').mockReturnValueOnce(mockSaveReturn);
const document = new modelPrototype(mockSaveReturn);
// Executing
await post(document.save())(response as Response);
expect(response.send).toHaveBeenCalledWith({ _id: '54759eb3c090d83494e2d804' });
expect(spySave).toHaveBeenCalled();
// Cleaning
spySave.mockClear();
});