Regarding ManyToManyFields in Django

Question

Suppose I make a model:

class A(models.Model):
       col = models.ManyToManyField(B)

Now after migration, this will create a table A_B with fields id,A_id,B_id. Now if I want to access the id of the table A_B, how to do that? And how to update the B_id of this table?

Why would you need `B_id`? You can just access B from the otherside like B.col_set (or similar)? Can you give your models *actual* names, so we understand what you are trying to model? — , Mar 09 '15 at 09:24

score 1 · Answer 1 · edited Mar 09 '15 at 09:19

1

You are looking for Through Relationships

class A(models.Model):
       col = models.ManyToManyField(B, through='C')

class C(models.Model):
      a = models.ForeignKey(A)
      b = models.ForeignKey(B)
      //any other extra field

Then you can filter or query C table.

For example, to print all C instances id:

for c in C.objects.all():
     print c.id

edited Mar 09 '15 at 09:19

GwynBleidD

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answered Mar 09 '15 at 07:31

levi

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Can't I do it without using a Through Relationship? – Priyansh Goel Mar 09 '15 at 07:50
@PriyanshGoel You will need raw SQL. – levi Mar 09 '15 at 21:44

score 1 · Answer 2 · answered Mar 09 '15 at 08:37

If you have an object of A:

A_obj.col.all()

This will return all the A_B objects related to that A

for ab in A_obj.col.all():
   print ab.id

This will print all the ids of the table A_B.

for ab in A_obj.col.all():
    ab.B_id = new id
    ab.save()

This will update your B_id with new id.

Regarding ManyToManyFields in Django

2 Answers2