动态模型的主键

Question

I'm building an app with dynamic models (following guidelines on [1]) to create an interface for an already existing database.

This is my function to create one of the models (in fact it has more fields and stuff, but this should be enough):

def create_model(name):
    # Meta class
    class Meta:
        pass

    # Set up a dictionary to simulate declarations within a class
    attrs = {'__module__': 'mymodule',
             'Meta': Meta,
             'fields': {
                'myID': models.CharField(max_length=2, db_column='myID', primary_key=True),
                '__unicode__': lambda self: self.myID,
                }
            }

    # Create the class, which automatically triggers ModelBase processing
    model = type(name, (models.Model,), attrs)

    admin.site.register(model, admin.ModelAdmin)

    return model

But when I try to access this model I get the following OperationalError :

(1054, "Unknown column 'database_table.id' in 'field list'")

It seems that Django is not taking into account my prymary_key definition for field myID and it's creating the default id field.

Am I missing something? Is it a ''bug'' related to dynamic model creation? Anyone knows a workaround for this? Thanks

[1] https://code.djangoproject.com/wiki/DynamicModels

Answer 1

Your fields and methods shouldn't be within the 'fields' dictionary in your attrs , but should be at the top-level of the attrs dict.

This:

attrs = {'__module__': 'mymodule',
         'Meta': Meta,
         'fields': {
            'myID': models.CharField(max_length=2, db_column='myID', primary_key=True),
            '__unicode__': lambda self: self.myID,
            }
        }

is functionally equivalent to this:

class MyModel(models.Model):
    class Meta:
        pass

    fields = { 
       'myID': models.CharField(max_length=2, db_column='myID', primary_key=True),
       '__unicode__': lambda self: self.myID,
      }

    def __unicode__(self):
        return self.myID

The ModelBase metaclass takes any field (actually any object with a contribute_to_class method) that's defined on the top-level of the class, and uses the contribute_to_class method on the field to 'add' it to the actual class. In this process, the field gets added to the fields dictionary, but you shouldn't define the fields dictionary before the actual metaclass is run. Your attrs should look like this instead:

attrs = {'__module__': 'mymodule',
         'Meta': Meta,
         'myID': models.CharField(max_length=2, db_column='myID', primary_key=True),
         '__unicode__': lambda self: self.myID,
        }

Answer 2

I'm building an app with dynamic models (following guidelines on [1]) to create an interface for an already existing database.

Are you sure this is the correct approach? Don't you want to use manage.py inspectdb to create models from that database instead? See:

https://docs.djangoproject.com/en/dev/howto/legacy-databases/