Question

From the example of Django Book, I understand if I create models as following:

from xxx import B

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

The Django would create a new table(A_B) beyond Table A, which has three columns:

• id
• a_id
• b_id

But now I want to add a new column in the Table A_B, thus would be very easy if I use normal SQL, but now anyone can help me how to do? I can’t find any useful information in this book.

Answer 1

It’s very easy using django too! You can use through to define your own manytomany intermediary tables

Documentation provides an example addressing your issue:

Extra fields on many-to-many relationships

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __unicode__(self):
        return self.name

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __unicode__(self):
        return self.name

class Membership(models.Model):
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    date_joined = models.DateField()
    invite_reason = models.CharField(max_length=64)