Thanks Mike!
My hesitation with directly manipulating that table is that there seems to be a lot of additional logic(validation, triggering signals, etc.) in add() besides insertion into the through table. However, I don't think I am relying on any of that functionality, so it should be ok to do it directly.
For context, I want to run migrations against a particular database. Any queries performed in RunPython operations need to specify that database's alias, otherwise they use the router which would direct them to use a different database than the one specified to the migrate command.
On Thursday, June 24, 2021 at 6:07:21 AM UTC+2 Mike Dewhirst wrote:
On 23/06/2021 7:18 pm, Jayanth Shankar wrote:
>
> Hi,
>
> I am trying to add items to a ManyToMany relationship, but I would
> like to make the operation use a particular database connection, like
> one might do when calling save() on a model instance. However, looking
> at the source for ManyToMany.add(), the function defaults to checking
> the router's for_write connection.
>
> class B(models.Model):
> ...
>
> class A(models.Model):
> b = models.ManyToManyField(B)
> ...
>
> ideally, I would like to modify the many to many field as follows...
> a = A(id=0)
> for b in B.objects.all():
> a.b.add(b, using="aconnection")
>
> but this does not work. As far Is there an alternative way to insert
> an object into the ManyToMany relationship(where a db alias can be
> specified) besides add?
Not sure I understand what you are trying to do but it is possible to
manage the many-to-many yourself. Django makes a table to connect A and
B without your needing to specify it as a model.
If you write a model to represent that table you can add other fields to
it and use its save() method to perform all sorts of magic.
For example, I have chemicals and products.
class Chemical(models.Model):
products = models.ManyToManyField(
"Product",
blank=True,
through="ChemicalProducts",
through_fields=("chemical", "product"),
related_name="products",
)
class ChemicalProducts(models.Model):
chemical = models.ForeignKey(
"Chemical",
on_delete=models.CASCADE,
related_name="base_chemical",
null=True,
blank=True,
)
product = models.ForeignKey(
"Product",
on_delete=models.CASCADE,
null=True,
blank=True,
)
# other fields and methods follow here
If I did not write the ChemicalProducts model, Django would make it
anyway. But because I have named the through table and it exists Django
doesn't bother.
This means I can write queries which insert m2m connections without
using 'add'.
However, I don't know because I haven't looked at the docs or the Django
source, whether you can use a callable for the 'through' table or even
specify the db_alias for it. I have only ever used the default myself.
HTH
Mike
>
>
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