On Aug 21, 2012, at 5:47 PM, Sergey V. wrote:
> Hi all,
>
> I've asked this question on stackoverflow:
> http://stackoverflow.com/questions/12031861/sqlalchemy-how-do-i-get-an-object-from-a-relationship-by-objects-pk/12032405
>
> Basically, I'm looking for a dict-like access to a relationship to be able to
> quickly retrieve items by some key (item's PK). To illustrate, a "normal"
> relationship is list-like:
>
> for book in library.books:
> print book.id # prints 10, 20, 30
>
> In addition, I'd like to be able to access "books" from an already-loaded
> relationship by their id:
>
> book1 = library.books.by_id(10)
> book2 = library.books.by_id(20)
> book3 = library.books.by_id(23) # raises KeyError
>
> while still being able to iterate over library.books in a list-like manner.
>
> I'm looking at MappedCollection
> (http://docs.sqlalchemy.org/en/latest/orm/collections.html?highlight=collection#custom-dictionary-based-collections)
> - would building a custom collection class be the right solution to the
> problem? Or is there something pre-built for this purpose in the depths of
> SQLAlchemy?
there's two variations here. One, which is definitely the quickest, is just
to use a @property that re-evaulates library.books as a dict:
@property
def books_by_id(self):
return dict((book.id, book) for book in self.books)
the other, the more "integrated" approach, is to use a collection_class that's
mostly a dict but just yields the values instead of the keys when iterated.
It might be nice if attribute_mapped_collection provided a simple argument to
pass in a "dict" subclass but apparently it doesn't have this yet, so just
subclass MappedCollection:
class IterateValuesDict(MappedCollection):
def __iter__(self):
return iter(self.values())
collection_cls = lambda: IterateValuesDict(keyfunc=lambda obj: obj.id)
class Library(Base):
# ...
books = relationship(Book, collection_cls=collection_cls)
>
> Thanks,
>
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