Question

Suppose I have a one-to-many relationship like this:

class Book(Base):
    __tablename__ = "books"
    id = Column(Integer)
    ...
    library_id = Column(Integer, ForeignKey("libraries.id"))

class Library(Base):
    __tablename__ = "books"
    id = Column(Integer)
    ...
    books = relationship(Book, backref="library")

Now, if I have an ID of a book, is there a way to retrieve it from the Library.books relationship, “get me a book with id=10 in this particular library”? Something like:

try:
    the_book = some_library.books.by_primary_key(10) 
except SomeException:
    print "The book with id 10 is not found in this particular library"

Workarounds I can think of (but which I’d rather avoid using):

book = session.query(Book).get(10)
if book and book.library_id != library_id:
    raise SomeException("The book with id 10 is not found in this particular library")

or

book = session.query(Book).filter(Book.id==10).filter(Book.library_id=library.id).one()

Reason: imagine there are several different relationships (scifi_books, books_on_loan etc.) which specify different primaryjoin conditions – manually querying would require writing individual queries for all of them, while SQLAlchemy already knows how to retrieve items for that relationship. Also, I’d prefer to load the books all at once (by accessing library.books) than issuing individual queries.

Another option, which works but is inefficient and inelegant is:

for b in library.books:
    if b.id == book_id:
        return b

What I’m currently using is:

library_books = {b.id:b for b in library.books}
for data in list_of_dicts_containing_book_id:
    if data['id'] in library_books:
        library_books[data['id']].do_something(data)
    else:
        print "Book %s is not in the library" % data['id']

I just hope there’s a nicer built-in way of quickly retrieving items from a relationship by their id

UPD: I’ve asked the question in the sqlalchemy mail list.

Answer 1

SQLAlchemy’s query object has with_parent method which does exactly that:

with_parent(instance, property=None)

Add filtering criterion that relates the given instance to a child object or collection, using its attribute state as well as an established relationship() configuration.

so in my example the code would look like

q = session.query(Book)
q = q.with_parent(my_library, "scifi_books")
q = q.filter(Book.id==10).one()

This will issue a separate query though, even if the my_library.scifi_books relation is already loaded. There seems to be no "built-in" way to retrieve an item from an already-loaded relation by its PK, so the easiest is to just convert the relation to a dict and use that to look up items:

book_lookup = {b.id: b for b in my_library.scifi_books}
book = books_lookup[10]