How to add column in ManyToMany Table (Django)

问题

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.

回答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)

回答2:

Under the hood, Django creates automatically a through model. It is possible to modify this automatic model foreign key column names.

I could not test the implications on all scenarios, so far it works properly for me.

Using Django 1.8 and onwards' _meta api:

class Person(models.Model):
    pass

class Group(models.Model):
    members = models.ManyToManyField(Person)

Group.members.through._meta.get_field('person').column = 'alt_person_id'
Group.members.through._meta.get_field('group' ).column =  'alt_group_id'

# Prior to Django 1.8 _meta can also be used, but is more hackish than this
Group.members.through.person.field.column = 'alt_person_id'
Group.members.through.group .field.column =  'alt_group_id'

来源：https://stackoverflow.com/questions/12567151/how-to-add-column-in-manytomany-table-django

标签

python

django

many-to-many