There was an error in my posted code, I meant:
'book_count': column_property(select([func.count(books.c.id)],
book_authors.c.author_id==authors.c.id).label('books_count'))
but that's irrelevant.
Using and_(), the *correct* definition is:
'book_count': column_property(select([func.count(books.c.id)], and_
(book_authors.c.author_id==authors.c.id,
book_authors.c.book_id==books.c.id)).label('books_count'))
Thanks for the help. By the way, I suggest adding this example to the
documentation; I know it would have saved me a lot of head-banging. I
suggest the following text tacked at the end of "SQL Expressions as
Mapped Attributes" in "Mapper Configuration":
For many-to-many relationships, use and_() to join the fields of the
association table to both tables in a relation:
mapper(Author, authors, properties={
'books': relation(Book, secondary=book_authors,
backref='authors'),
'book_count': column_property(select([func.count(books.c.id)], and_
(book_authors.c.author_id==authors.c.id,
book_authors.c.book_id==books.c.id)).label('books_count'))
})
On Jul 22, 4:10 pm, "Michael Bayer" <[email protected]> wrote:
> bojanb wrote:
> > mapper(Book, books)
> > mapper(Author, authors, properties={
> > 'books': relation(Book, secondary=book_authors,
> > backref='authors'),
> > 'book_count': column_property(select([func.count(books.c.id)],
> > book_authors.c.author_id==books.c.id).label('books_count'))
> > })
>
> the subquery within the column property is not joined to the parent table
> "authors". It needs to join book_authors back to the authors table using
> AND. the subquery will automatically correlate outwards to the "authors"
> query so "authors" would only render in the outermost FROM (which is what
> you want).
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