It doesn't make sense for the Samurai to be out of all rooms. If it's
because he is dead you should create a room for that, such as "Graveyard" or
something. However, I think it would work to include "blank = True" in the
declaration. This would allow the field to have a null value.
-Tim
On Fri, Feb 19, 2010 at 2:32 AM, Le Quoc Viet <viet.aikid...@gmail.com>wrote:
> Hi Tim,
>
> Thanks for reply. I mean if I want Samurai to be out of all rooms?
> ForeignKey disallows Null. Should I do like this: Create a default dummy
> Room and assign it by default?
>
> Viet.
>
>
> Timothy Kinney wrote:
>
> I don't understand your question. The Samurai requires one room by the
> declaration:
>
> class Samurai(models.Model):
> ...
> room = models.ForeignKey("Room")
> ...
>
> Since it is a ForeignKey, it can only accept one value and the value cannot
> be None. Therefore, all samurai will always be in only one room at a time.
>
> Hope this helps.
>
> -Tim
>
> On Fri, Feb 19, 2010 at 12:45 AM, Le Quoc Viet <viet.aikid...@gmail.com>wrote:
>
>> Thanks for explanation. What if Samurai can be outside all Rooms and can
>> be at most at 1 Room at a time?
>>
>> Regards,
>> Viet.
>>
>>
>>
>> Peter Herndon wrote:
>>
>> On Feb 18, 2010, at 11:47 PM, Timothy Kinney wrote:
>>
>>
>>
>> Here's the system I'm currently using that doesn't work too well...
>>
>> Samurai (id, name)
>>
>> Room (id, name, ForeignKey(Samurai), ForeignKey(Province))
>>
>> Province (id, name)
>>
>>
>> Can someone suggest which relationships to use to get the results that
>> I want and explain why the relationship helps to accomplish that?
>>
>>
>>
>> If a given samurai must be in a room, and a room can hold multiple samurai,
>> your models should instead be:
>>
>> Samurai (id, name, ForeignKey(Room))
>>
>> Room (id, name, ForeignKey(Province))
>>
>> Province (id, name)
>>
>> With this set of models, a single Samurai has an FK reference to the room in
>> which he finds himself. The Room, very similarly, has a reference to the
>> Province in which it is located. A Province can hold many Rooms, and a Room
>> can hold many Samurai. In Django's ORM models, a given instance of the Room
>> model (e.g., "room = Room.objects.get(pk=1)") will have a method that allows
>> access to all Samurais residing in that Room. So, "room.samurai_set" will
>> give you the Room's Samurais. Similarly, for a given province, you can
>> retrieve all rooms via "province.room_set". Relevant Django docs are here:
>> http://docs.djangoproject.com/en/1.1/ref/models/relations/#ref-models-relations
>>
>> In a view, assuming you are given a Samurai, you can get to the room name
>> via samurai.room.name, and the province name by samurai.room.province.name.
>> If, in your view, you pass the Samurai object to the template, your template
>> can perform that same dereference and thus you can present to the user the
>> room name and province name in your HTML. Relevant Django docs here:
>> http://docs.djangoproject.com/en/1.1/topics/templates/#variables
>>
>> For the canonical book on relational database theory, I'd suggest
>> "Introduction to Database Systems", by Chris Date. Not for the faint of
>> heart, the math-challenged, the practical or those short of time, but it
>> will explain relational theory in detail. Great detail. Ye be warned. ;)
>>
>> I hope this helps, though the Date book probably won't, at least not without
>> long and sustained study. There are likely other books on RDBMSs that folks
>> can recommend. Other than that, my apologies, it's late and I no longer
>> have a functioning brain.
>>
>> ---Peter Herndon
>>
>>
>>
>>
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