Hi,
I'm new to django and have been working hard to find "the right way"
or at least "a good way" to do things. I'm running into a few
problems though that I can't get answers for.
The following post describes my situation very clearly. It may be
long-winded but hopefully it's unambiguous. I'm sure there'll be
nothing difficult to the experienced django user here, but some
answers could be helpful to me and other beginners.
In my django model I have the following classes: Person and
BankAccount. There are other model types too, but these should
suffice to illustrate my problem.
Person:
person_id
first_name
surname
BankAccount:
bank_account_id
person_id (ForeignKey)
bank_code
account_number
A Person may have ZERO or ONE BankAccounts.
I want to be able to create or edit the details for a Person
(including their associated BankAccount, should it exist) on the one
html form. I've created some ModelForm types to help with this:
class PersonForm(ModelForm):
class Meta:
model = Person
class BankAccountForm(ModelForm):
class Meta:
model = BankAccount
exclude = ('person',) #Avoid the dropdown menu
I've been advised (on IRC #django) that "create new" should be handled
at one URL and "edit existing" at a separate URL. I handle the URLs
at different view functions. So in url.py:
url(r'person/add/$', 'person_add', 'person_add'),
url(r'person/(?P<person_id>\d+)/$', 'person_edit',
name='person_edit'),
The "person_add" view function is easy to implement. For request.GET
for I simply create a new PersonForm and a new BankAccountForm and
expose them to the template via the context. The request.POST is easy
too - however I only save the BankAccount data if non-blank data was
supplied.
The "person_edit" view function raises some problems though:
1) I may be editing a Person that has no associated BankAccount. So
basically I'm supplying a form with "edit existing" functionality for
the Person model and "create new" functionality for the BankAccount.
This doesn't cause a problem in itself, but it does break the paradigm
of separating "create new" and "edit existing" functionality - if such
a paradigm is indeed correct.
2) When a GET request to /person/5/ is received, I retrieve the
relevant Person from the db. In a separate db query I then have to
try to get an associated BankAccount. If I do:
BankAccount.objects.get(person=person_id)
and the BankAccount doesn't exist a DoesNotExist exception is
thrown... This really surprised me. Thinking about it from a db query
perspective I would have expected an empty list. I notice however if
I do:
BankAccount.objects.filter(person=person_id)
I receive an empty list when there is no match. I presume this is by
design, I just wonder what the logic of that design is. Any ideas?
3) When a POST request to /person/5/ is received the URL tells me I'm
editing Person "5", but I also need to know if I'm editing or creating
a BankAccount too. I can obviously query the BankAccount table for a
record matching person_id=5, but again there is a db roundtrip
overhead here. Alternatively, I think I could have included a hidden
bank_account_id field when I received the GET request for /person/5/
so that upon POST I would "know" the bank_account_id - with some
security concerns though. What is best-practice in this situation?
4) You might notice that the relationship between Person and
BankAccount is modelled as a One-To-Many, however my business logic
requires One-To-(Zero or One). I wondered if I could use the
models.OneToOneField for this, but the documentation seemed to imply
that django would expect exactly one record on BOTH sides of
relationship, so it may not be suitable for the One-To-Zero case.
Again, if anyone has advice on what I should actually do here, please
let me know.
Obviously this is a simple example and I could code a solution that
would work. However, my real problem involves several similar
relationships on the one form. The POST handling code could get very
hairy if I don't do it well.
If you got this far, thanks for taking the time to read this.
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