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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