Question

I’m re-asking this question but with a different framework this time. I have two Models: User and Book with a M2M-relation. I want Book to have an attribute “read” that is True when the relation exists. Is this possible in SQLAlchemy?

Answer 1

Take a look at SQL Expressions as Mapped Attributes. Something like this should do the job for you:

Book.read = column_property(
        select(
            [func.count(user_to_book_table.c.user_id)],
            user_to_book_table.c.book_id == book_table.c.id
        ).label('read')
    )

Even though it is not Boolean, you can still use it in the IF statements correctly:

if mybook.read:
    print 'very popular book indeed'

Alternatively you can just add a computed (read-only) property on the Book object, but this will load all the Users into your session:

@property
def read(self):
    return len(self.books)!=0