Specifying DB table names for intermediate table in an M2M relation?
Geta
intermediate table for a ManyToManyField? I have a legacy database,
with two tables and an intermediate table for the M2M relationship
between them -- all these are pretty similar to what Django would
create, but the table and column names are different. I know you can
specify META.db_table to specify database table names and db_column for
field names, but I couldn't see where/if this could be set for a
ManyToManyField -- is it possible?
Thanks,
Geta
Russell Keith-Magee
I know you can
specify META.db_table to specify database table names and db_column for
field names, but I couldn't see where/if this could be set for a
ManyToManyField -- is it possible?
There isn't a simple way to do this - however, if you're feeling adventurous, there are a few things that might be worth trying.
The m2m table name is determined by the m2m_db_table() method on the ManyToManyField itself (from django.db.models.fields.related.py). Similarly, the m2m_column_name() and m2m_reverse_name() methods on the ManyToManyField determine the column names that are used on the m2m table. These three functions are curried versions of more generic functions (_get_m2m_db_table, et al). At present, they are hard coded to use the field and related model names as the basis for naming of the m2m table.
If you were to subclass ManyToManyField and override the _get_m2m versions of the functions, or modify the m2m_...() functions on a field instance, you might find that you can point the m2m relation at your existing table.
As a more general solution - your idea is reasonable enough, and shouldn't be too hard to implement. I would suggest submitting it as an enhancement request.
Yours,
Russ Magee %-)
Malcolm Tredinnick
Not yet.
There's a ticket I stumbled across yesterday that asks for this, too
(#785). I think it's not an unreasonable idea, so we should add
something like that in the near- to medium-term future.
Malcolm
Marc de Graauw
Geta wrote:
> What's the easiest way to have Django use an existing table for the
> intermediate table for a ManyToManyField?
In PostgreSQL you could try to:
- let Django create its m2m table
- drop it
- recreate it as a view on your original m2m table
- create an rule 'AS ON INSERT ... DO INSTEAD"
A small script which shows something like this:
/* Your original m2m table */
create table t (k int, c1 int, c2 int);
/* The view, which should copy Django's m2m table */
create view v as select k as id, c1 as t1_id, c2 as t2_id from t;
/* A rule which redirects inserts to the original m2m table */
create rule r as on insert to v
do instead
insert into t values (NEW.id, NEW.t1_id, NEW.t2_id);
/* insert some values in the view */
insert into v values (1, 1, 1);
insert into v values (2, 2, 2);
insert into v values (3, 3, 3);
/* inserts have actually gone in the original m2m table */
select * from t;
You would also need rules for updates and deletes on the view.
On MS SQLServer views are updatable to a certain extent by default, so
something similar would work there.
Of course the Django-generated m2m table and the original one have to
be pretty similar for this to work.
Cheers,
Marc