How to test mongoose in NestJS Service? - node.js

Question

I would like to test getFund() method from my service. I use NestJS that uses jest by default.
I have no idea how to test this line with jest: return await this.fundModel.findById(id);. Any idea?
import { Injectable } from '#nestjs/common';
import { Model } from 'mongoose';
import { Fund } from '../../funds/interfaces/fund.interface';
import { InjectModel } from '#nestjs/mongoose';
#Injectable()
export class FundService {
constructor(
#InjectModel('Fund')
private readonly fundModel: Model<Fund>,
) {}
/*****
SOME MORE CODE
****/
async getFund(id: string): Promise<Fund> {
return await this.fundModel.findById(id);
}
}
Edit
Thanks to slideshowp2 answer, I wrote this test.
describe('#getFund', () => {
it('should return a Promise of Fund', async () => {
let spy = jest.spyOn(service, 'getFund').mockImplementation(async () => {
return await Promise.resolve(FundMock as Fund);
});
service.getFund('');
expect(service.getFund).toHaveBeenCalled();
expect(await service.getFund('')).toEqual(FundMock);
spy.mockRestore();
});
});
The problem is that I get this result in my coverage report:
When I hover the line I get statement not covered.

Answer 1

There is only one statement return await this.fundModel.findById(id); in your getFund method. There is no other code logic which means the unit test you can do is only mock this.fundModel.findById(id) method and test
it .toBeCalledWith(someId).
We should mock each method and test the code logic in your getFund method. For now, there is no other code logic.
For example
async getFund(id: string): Promise<Fund> {
// we should mock this, because we should make an isolate environment for testing `getFund`
const fundModel = await this.fundModel.findById(id);
// Below branch we should test based on your mock value: fundModel
if(fundModel) {
return true
}
return false
}
Update
For example:
describe('#findById', () => {
it('should find ad subscription by id correctly', async () => {
(mockOpts.adSubscriptionDataSource.findById as jestMock).mockResolvedValueOnce({ adSubscriptionId: 1 });
const actualValue = await adSubscriptionService.findById(1);
expect(actualValue).toEqual({ adSubscriptionId: 1 });
expect(mockOpts.adSubscriptionDataSource.findById).toBeCalledWith(1);
});
});
The test coverage report: