This is my first time building an app with django and so far I've been
more than happy with it. However, I can't seem to figure out how to
sufficiently optimize queries for ManyToMany relationships. Here is a
model to illustrate my problem:
class Book(models.Model):
name = models.CharField(max_length=255)
class Reader(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
books_read = models.ManyToMany(Book, relatedName='read_by')
Now I simply want to list every reader along with the books that
they've read. For example:
Reader Books Read
----------
----------------------------------------------------------
Claire Smith Great Expectations
Jane Eyre
Wuthering Heights
Dan Smith Great Expectations
War and Peace
Adventures of Sherlock Holmes
Bob Smith Adventures of Sherlock Holmes
The Call of the Wild
War and Peace
.... ....
Ideally select_related() would work with ManyToMany relationships as
well as ForeignKey relationships but I can see that this would take
the model query API to a whole other level of complixity so I can
understand why it hasn't been done yet. I'm used to ORM libraries like
Java's Hibernate that can fetch entire graphs of objects with a
minimum number of queries but I'm more than happy to do a little extra
work to avoid the complexity of something like Hibernate.
However, I would hope that I could fetch the necessary data to
instantiate the object graph for any number of readers with only 2
queries. The queries would look like:
readers = Reader.objects.filter(last_name='Smith');
books = Book.objects.filter(read_by__last_name='Smith');
These two queries get me all the data I need but I still don't have
the connectivity information to know which readers have read which
books. In order to get this piece of info, it looks like I'll have to
drop into the native sql world and fetch the info directly from the
many-to-many table created by django. Is this what necessary to do the
database optimization or am I missing something? It's not so bad in
this simple case but as the graph of interesting objects gets bigger,
I'm afraid I may have to write a lot of this type of code.
Thanks,
--Dan
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