Question

As a project to figure out Django I’m trying to build a small game.

A player has a base. A base has several type of items it can harbor. (Vehicle, Defense, Building).

I have 3 static tables which contain information for the first level of each item (in the game these values are used in formulas to calculate stuff for upgrades). I’ve used a sequence to insert all these items in these different tables so the ID’s are unique across tables.

To keep track of what items the player has per base I have a table ‘Property’. I want to use a single field as a reference to the ID of an item and trying to get this done with the Django models.

Warning: my knowledge about Django models are pretty limited and I’ve been stuck with this a few days now.

Is this possible and if so how can it be done?

I tried using annotations on the save method to change the value of a field by overwriting the field with the id of that object before trying to query the object by id when trying to ‘get’ the object, however I can’t get past the obvious restriction of the model when defining that field as an Integer – I hoped it wouldn’t validate until I called save()

def getPropertyItemID(func):
    """
    This method sets the referral ID to an item to the actual ID.
    """

    def decoratedFunction(*args):
        # Grab a reference to the data object we want to update.
        data_object=args[0]

        # Set the ID if item is not empty.
        if data_object.item is not None:
            data_object.item=data_object.item.id

        # Execute the function we're decorating
        return func(*args)

    return decoratedFunction

class Property(models.Model):
    """
    This class represents items that a user has per base.
    """

    user=models.ForeignKey(User)
    base=models.ForeignKey(Base)
    item=models.IntegerField()
    amount=models.IntegerField(default=0)
    level=models.SmallIntegerField(default=0)

    class Meta:
        db_table='property'

    @getPropertyItemID
    def save(self):
        # Now actually save the object
        super(Property, self).save()

I hope you can help me here. The end result I’d like to be able to put to use would be something like:

    # Adding - automatically saving the ID of item regardless of the class 
    # of item
    item = Property(user=user, base=base, item=building)
    item.save()

    # Retrieving - automatically create an instance of an object based on the ID 
    # of item, regardless of the table this ID is found in.
    building = Property.objects.all().distinct(True).get(base=base, item=Building.objects.all().distinct(True).get(name='Tower'))
    # At this point building should be an instance of the Building model

If I’m completely off and I can achieve this differently I’m all ears 🙂

Answer 1

I think you are looking for a Generic Relationship:

class Property(models.Model):
    user=models.ForeignKey(User)
    base=models.ForeignKey(Base)
    content_type = models.ForeignKey(ContentType) # Which model is `item` representing?
    object_id = models.PositiveIntegerField() # What is its primary key?
    item=generic.GenericForeignKey('content_type', 'object_id') # Easy way to access it.
    amount=models.IntegerField(default=0)
    level=models.SmallIntegerField(default=0)

This lets you create items as you mentioned, however you would probably need to look at a different way of filtering those items out.