over 7 years ago
Two small issues I run into when dealing with OneToOneField and iterating through a model's fields.
One to One Fields
class A(Model):
...
class B(Model):
a = models.OneToOneField(A, primary_key=True)
When models are defined like above, you can grab a reference to B this way:
a_instance = A.objects.get(id=1)
b_instance = a_instance.b
Since B's primary key equals to A's primary key, when Django's ORM creating B in the database, it will not create an 'id' column. Instead, a column named 'A_id' is created. Thus, when ever you want to access B's primary key, you cannot do:
b_instance.id # AttributeError: 'b_instance' object has no attribute 'id'
You have to use b_instance.pk instead.
Model's _meta field
I once had a task which needs to determine if any field within a model instance is empty. To do it, I need a way to loop through all of the fields defined in the instance first:
# _meta allows you to access the fields defined in the model instance
for field in model_instace._meta.fields:
...
Here is the code for determining if a model instance is empty:
def is_model_instance_empty(model_instance):
# find all of the field's name that are not foreign keys or one to one fields
# many to many relationship should be added to make it complete
field_names = [field.name for field in model_instance._meta.fields if (field.get_internal_type() != 'OneToOneField' and field.get_internal_type() !='ForeignKey')]
counter = 0
for field_name in field_names:
if not model_instance.__dict__[field_name]:
counter += 1
return counter == len(field_names)
