Why is filtering by foreign keys fields not allowed?

[Abdeldjalil-H]

Let's say we have two tables

class Author:
    id = ormar.Integer(primary_key=True)
class Book
    ...
    author = ormar.ForeignKey(Author)

Then the following queries should be equivalent

Book.objects.get(author=5)

Book.objects.get(Book.author==5)

However the second query raises the following exception

AttributeError: Cannot filter by Model, you need to provide model name

This is caused by self._check_field() inside the method FieldAccessor._select_operator. An alternative approach is to replace the last query by

Book.objects.get(Book.author.id==5)

but this will do a join query, which is different from the first query. My question is, why filtering by foreign keys fields is not allowed?