On Thu, 2009-02-12 at 17:57 -0800, azymnis wrote:
> I am in a situation where I want to create a model instance from
> another object. For this reason, I am trying to override the Model
> constructor so that it can take an object as an argument. However,
> this does not seem to work properly, since after I save, I cannot
> access the data with the object manager. Here is an example. Suppose
> my models.py code is:
>
>
> from django.db import models
>
> class Person(models.Model):
> first_name = models.CharField(max_length=40)
> last_name = models.CharField(max_length=40)
>
> def __init__(self,datadict):
The problem is that you have changed the signature of the __init__
method by doing this. And your code is not the only code that constructs
instances of this class. In particular, the queryset code wants to try
to create Person instances and it needs to pass in a tuple of arguments.
The QuerySet.iterator() code will be trying to call Person(*(1, 'Rob',
'Smith')), or something similar.
I'm not entirely sure right now why the exception this generates gets
swallowed, but it doesn't particularly matter.
You cannot change the signature of __init__ like that. It must be able
to accept arbitrary positional and keyword arguments (*args, **kwargs).
That's pretty normal subclassing practice: when subclassing something,
you should only extend the signature of the base-class with optional
parameters. That way, your subclass can be used wherever the base class
would be used. You could rewrite your code like this to make it work:
def __init__(self, *args, **kwargs):
if "datadict" in kwargs:
ddict = kwargs["datadict"]
kwargs = {
"first_name": ddict["first_name"],
"last_name": ddict["last_name"],
}
super(Person, self).__init__(*args, **kwargs)
There are a few variations on that code which are possible, depending on
how robust you want to be in checking that only "datadict" is passed in.
That's up to you and your usage practices; there's no particularly
unique right answer.
Regards,
Malcolm
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