Cannot chain existing mongoose query object

[newmanw]

const query = Model.find(conditions, fields, options);
if (someCondition) {
  query.populate('someField)
}
query.exec(callback);

Trying to test populate is called

sinon.mock(Model)
  .expects('find')
  .chain('populate).withArgs('someField')
  .chain('exec')
  .yields(null, {
    foo: 'bar'
  });

I get the following error:

Uncaught TypeError: Cannot read property 'emit' of undefined

If I remove the chain for 'populate' it works just fine.

Debugging further it looks like the yields is not returning the mock object causing a failure further in the test.

[newmanw]

Looks like I can modify the code to reassign the query as a work around.

let query = Model.find(conditions, fields, options);
if (someCondition) {
  query = query.populate('someField)
}
query.exec(callback);