When comparing two mongoose models whose _id or discriminated docs arent equal, the library returns no differences.

Steps to reproduce: (pseudo code)

1.Find a document

const original = await exampe.findOne(...);
// original.referencedDoc has value = new ObjectId("667c39c3ef67892912314323");

2. Perform an update and find document again

await original.update({ referencedDoc: new ObjectId("64c7cda1db76ef25cfc5f901"); })
const latestDocument = await exampe.findOne(...);

3. Compare differences between original and latestDocument.

const diffs = diff(original, latestDocument);
console.log('diff', diffs) 

Output: (prints no differences)
{}

Expected output: (should print the difference)

{
 referencedDoc: new ObjectId("64c7cda1db76ef25cfc5f901");
}

This bug may be fixed by changing the isEmpty utility function:

const isEmpty = o => Object.keys(o).length === 0;

It is returning zero because "ObjectId" is a class without "keys", but it can be readed as a string perfectly.