How can I stub mongodb's addCursorFlag in mocha test?

Question

I have this code to unit test:

    collection.find({
      result: { $exists: false }
    }, {
      timeout: false,
      maxTimeMS: 1800000,
    })
      .addCursorFlag('noCursorTimeout', true)
      .stream()
        .
        .
        .

How can I stub the .addCursorFlag('noCursorTimeout', true) part using sinon? Here is the unit test that worked, before adding in the addCursorFlag:

      collectionStub.find = sinon.stub().returns({
        stream: () => new mocks.stream.ReadableMock(false, null, 'mongo error')
      });

      jobRunner.on('error', (updatedJob) => {
        expect(updatedJob).to.deep.equal({
          ...job,
          status: 'error',
          error: 'mongo error',
        });

        done();
      });

      jobRunner.run();
    });

Answer 1

It's kind of tricky but if you observe the method chaining after collection.find, you have a call to addCursorFlag which then returns an object from where you call the stream method. You need to structure your collection stub in a similar way:

collectionStub.find = sinon.stub().returns({
        // addCursorFlag is a function that returns an object
        addCursorFlag: () => ({
          // stream is a method in the returned object 
          stream: () => new mocks.stream.ReadableMock(false, null, 'mongo error')
        )}
      });