如何避免django“与相关的m2m字段冲突”错误？

Question

I have a lot of models with voting functionality, so I created a structure like this:

class Voteable(models.Model):
    likes_balance = models.IntegerField(default=0, editable=False)
    votes = models.ManyToManyField(User, blank=True, editable=False)
    likes = models.ManyToManyField(User, blank=True, editable=False)

    class Meta:
        abstract = True

class Item(Voteable):
    title = models.CharField(max_length=20, db_index=True)
    description = models.TextField(max_length=1000)
    contact_user = models.ForeignKey(User, null=True, blank=True)

    class Meta:
        abstract = True

class Movie(Item):
    cover = models.ImageField(upload_to='images/covers/')

class Car(Item):
    seller = models.CharField(max_length=50)

When I try to create tables with "python manage.py syncdb" I get error message:

Accessor for m2m field 'likes' clashes with related field 'User.movie_set'. Add a related_name argument to the definition for 'likes'.

Of cause I have much more fields in Item class, so don't want to copy all of them to all subclasses and just set related_name like suggested in error.

Any suggestions how to deal with it?

Answer 1

I found a solution in Django documention .

It's possible to write in abstract models things like this: related_name="%(app_label)s_%(class)s_related"

Answer 2

Normally if you add a related_name as suggested in your M2M definition, it should work :

class Voteable(models.Model):
    likes_balance = models.IntegerField(default=0, editable=False)
    votes = models.ManyToManyField(User, blank=True, editable=False, related_name='votes')
    likes = models.ManyToManyField(User, blank=True, editable=False, related_name='likes')

    class Meta:
        abstract = True

It's because by not doing so, Django will add two user_id in the Voteable table, resulting in a clash because there is twice the same column name. Adding a related_name force Django to use the given related_name instead of the {Foreign Table Name}_id column name.

Hope this helps.