Hi all,
I've just finished the polishing the first part of Alex's GSoC work to
add Multi-db, which I'd like to commit. This is a call for feedback.
This first patch isn't directly related to multi-db, but it is a
prerequisite that will allow the multi-db code to function. The goal
of this patch is to remove all uses of raw SQL from related.py. This
means that all M2M and FK operations are performed using Django
queries, rather than using cursors and manually constructed SQL. This
is needed so that multi-db queries have complete control over the SQL
that is generated by Django. As an aside, this is also a prerequisite
for the non-SQL backend work, as non-SQL backends obviously can't use
a SQL cursor.
This is being tracked as ticket #10109. I've uploaded the most recent
patch to that ticket. Code is also available from Alex's github branch
(details on the ticket).
My intention is to commit this mid next week. If you have any
objections (either to the patch, or my intended commit schedule), now
is the time to raise them.
Here are the fine details for anyone interested:
First - an example set of models:
class Author(Model):
name = CharField()
class Book
name = CharField()
authors = ManyToManyField(Author)
Historically, accessing mybook.authors resulted in the creation of a
cursor and the execution of some hand-crafted SQL to retrieve data
from the myapp_book_authors table.
Using this patch, the authors m2m relation is now backed by an
automatically generated m2m model. You don't have to do anything to
get this model - it is created during the contribute_to_class
processing for the m2m field.
The automatically generated m2m model is the equivalent of the following:
class Book_authors:
book = ForeignKey(Book)
author = ForeignKey(Author)
This model is then used as a 'through' model in the m2m-intermediate
sense. All requests to add/remove from the m2m relation are handled by
creating/destroying instances of the m2m model.
The auto-created model is available in the model index if you ask for
it explicitly, but it won't be returned by loader.get_models() unless
you explicitly ask for the auto-created models (using
loader.get_models(include_auto_created=True) )
Signals for the m2m model have been silenced - you won't get a
create/destroy signal every time you add/remove an m2m object.
The other interesting feature is that the reverse relations (the _set
objects that would normally exist on Book and Author) have been
hidden. This is achieved by appending '+' to the related names. The
use of '+' to hide relations has been in Django for a while (to mask
the reverse relation for symmetric m2m relations with self) - this
patch expands the use of '+' on a field name to a more general
field-hiding capability.
As a result of using normal Django models for the m2m relation, we can
deprecate a lot of the special handling for m2m models in the model
creation code in the backends. With this patch, an m2m table is just a
normal model with two foreign keys, so we can use normal table
creation code.
Some interesting fallout onto other tickets:
* #11795 (allowing admin inlines for m2m objects) becomes a 2 line
fix, plus documentation. Since this patch makes m2m tables fully
fledged Django objects, you can reference them in the models= line of
a TabularInline. m2m inlines are actually possible without any change
on top of this patch - but with two extra lines, it becomes a lot
easier to use.
* #5537 becomes a purely documentation fix. If you use a related_name
of '+' on your model, the related object is hidden. The only question
here is whether we want to document this change - so far the '+' thing
has been an internal implementation issue, so we need to decide if we
want to make this official.
So - that's the patch. Feedback welcome.
Yours,
Russ Magee %-)
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