爲什麼South決定刪除這個m2m表？

我想添加一列到現有的模型，並在運行「schemamigration --auto」後產生的遷移嘗試刪除另一個表！爲什麼會這樣做？爲什麼South決定刪除這個m2m表？

我的模型（簡體） -

class A(Model): 
    a = CharField() 
    b = BooleanField(default=False) # <--- this is the new column 

class B(Model):    # <---- this is the table South wants to delete 
    c = CharField() 
    d = ManyToManyField(A, through='C')

結果[遷移

def forwards(self, orm): 
    # Removing M2M table for field d on 'B' 
    db.delete_table('B_d') # <------- Why is that? 
    # Adding field 'b' 
    db.add_column('A', 'b', self.gf('django.db.models.fields.BooleanField')(default=False, blank=True), keep_default=False)

來源

2011-02-13 Iftah