$exist does not seem to work in find() method

[sadeghhosseini]

Hi,
I am using mongoose 6.3.4. $exists does not seem to work within find() method.

This is my code.

Author.find({
     $or: [
        {
            books: {
                $exists: true,
                $not: { $size: 0 },
            }
        },
        {
            books: {
                $exists: false,
            }
        },
    ],
})

I am trying to get author documents which either does not have the field books defined on them or the book field is defined on them but it is an empty array.