使用Django查询获取相关模型

Question

Can you please help me with Django query. I am new in this stuff and I am trying to optimize the queries in my site. Below is the current case: I use the standard User model as base model and have 3 types of users, which are related with it: A, B and C. "User" can be related with only one of these 3 model. Currently I have a list with instance from Model D, which is related with User as well. Below is my code

D = D.objects.all()
for element in D:
try:
     results.append(element.user.A)
except:
    try:
        results.append(element.user.B)
    except:
        results.append(element.user.C)

As you already noticed, this solution is not the best one in this case(and maybe this is the worst). I appreciate any help. Thanks in advance.

Edit: Here is and my models:

class A(models.Model)
    facebook_id = models.CharField(max_length=20, blank=True, null=True)
    user = models.OneToOneField(User, unique=True)
    ...

class B(models.Model)
    user = models.OneToOneField(User, unique=True)
    ...

class C(models.Model)
    user = models.OneToOneField(User, unique=True)
    ...

class D(models.Model):
    user = models.ForeignKey(User)
    F = models.ForeignKey(F)

Answer 1

I actually had the very same issue some time ago, and if I were to do it again, I'd follow remy_g's hint no matter how different the models are.

That being said, you could use hasattr() instead of the messy try / except nest.

http://docs.python.org/2/library/functions.html#getattr

What it would look like :

D = D.objects.all()
for element in D:
    if hasattr(element.user, "A"):
        results.append(element.user.A)
    elif hasattr(element.user, "B"):
        results.append(element.user.B)
    elif hasattr(element.user, "C"):
        results.append(element.user.C)

You can improve this to your liking but you get the idea...

Hope this helps,

Regards,