[storm] Lazy value loading

[Thomas Hervé]

Le vendredi 24 octobre 2008 à 14:16 +0200, Adrian Adamiak a écrit :
> Hi. I've got a question about lazy value loading.
> 
> Let's say that I have an ORM mapping for table 'book':
> 
> class Book(object):
>     __storm_table__ = "book"
>     id = Int(primary=True)
>     author = Unicode()
>     title = Unicode()
>     contents = Unicode()     # this can be big
> 
> Sometimes I need to display a list of titles and authors of all books, so I use:
> 
> books = store.find(Book)
> 
> Unfortunately this produces query which loads all fields, including
> 'contents', which is unnecessary and takes a lot of time. I'd like to
> make the 'contents' field lazy, i.e.
> 
>  contents = Unicode(lazy=True)    # (in Book class definition)
> 
> so that it would load only after I access this field (i.e.
> books[0].contents). Is it possible?

The way I'd do it would be like that:

class Book(object):
    __storm_table__ = "book"
    id = Int(primary=True)
    author = Unicode()
    title = Unicode()
    
    @property
    def contents(self):
        return Store.of(self).execute("SELECT contents FROM book WHERE
id = ?", (self.id,)).get_one()[0]

This way you're sure that you don't get through all Python code for
nothing. It's in no way optimal, but it works.

-- 
Thomas