Re: Django - Alternative to using NULLs? (for integer and FK fields).

On Tue, Nov 30, 2010 at 4:11 AM, Lachlan Musicman <datakid@gmail.com> wrote:
> On Tue, Nov 30, 2010 at 12:28, Victor Hooi <victorhooi@gmail.com> wrote:
>> Hi,
>>
>> I'm wondering what the community's stance on using NULL in Django is?
>>
>> Say for example you have:
>>
>>    class Person(models.Model):
>>        street_address = models.CharField(max_length=50, blank=True)
>>        suburb = models.CharField(max_length=30)
>>        postcode = models.IntegerField()
>>        state = models.CharField(max_length=3)
>>        email = models.EmailField()
>>        mobile_phone_number = models.IntegerField(max_length=12)
>>        home_phone_number = models.IntegerField(max_length=10,
>> null=True, blank=True)
>>        work_phone_number = models.IntegerField(max_length=8,
>> null=True, blank=True)
>>
>>       spouse = models.ForeignKey('self', null=True, blank=True)
>>       children = models.ManyToManyField('self', null=True,
>> blank=True)
>>
>> For string fields like street_address, I can make these "blank=True",
>> and Django will store an empty string if the user leaves it blank.
>>
>> However, for integer fields like home_phone_number and
>> work_phone_number, I've had to make these "null=True" for the case
>> where somebody doesn't supply them (i.e. they're meant to be optional,
>> mobile is required).
>>
>> However, is there a better way of handling this case? (assuming I want
>> to keep these fields as integers).
>
>
> Is it possible to know why you would want to keep them as integers?
> Given that there are no mathematical functions that you would want to
> apply to them....
>
>
>> What about in the case of optional foreign keys (spouse and children)
>> - is there a better way of handling these, without using NULLs?
>
> As I understand it, foreign keys are kept in the db as follows:
>
> 1. table_Person
> 2. table_Person_children
> 3. table_Person_spouse

You understand it incorrectly. A foreign key on fooapp.FooModelA to
fooapp.FooModelB would be modelled in the database as an
integer/foreign key field (depending on engine) called foomodelb_id on
table fooapp_foomodela.

>
> table 2 has three columns: id, Person, Children
> table 3 has three columns: id, Person, Spouse
>
> or something to that effect.
>
> Therefore, if there is no Spouse or Child, there is no entry for
> Person in tables 2 or 3.

You are describing an m2m relationship, not a foreign key.

Cheers

Tom

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