gistfile1.py

from django.db import models
from django.db.models.fields import CharField

class ModelField(CharField):
    __metaclass__ = models.SubfieldBase

    def __init__(self, **kwargs):
        kwargs['blank'] = True
        kwargs['max_length'] = 255
        super(ModelField, self).__init__(**kwargs)

    def pre_save(self, obj, add):
        value = getattr(obj, self.attname)
        if value is None:
            return value
        else:
            return self._obj_to_data(value)

    def get_db_prep_value(self, value, connection, prepared=False):
        if prepared:
            return self._obj_to_data(value)

        if value is None: return None
        elif isinstance(value, basestring): return value
        else: return self._obj_to_data(value)

    def _obj_to_data(self, obj):
        return "{0}.{1}|{2}".format(obj.__module_, type(obj).__name__, obj.pk)

    def to_python(self, value):
        from django.core.exceptions import ValidationError
        if not isinstance(value, basestring):
            return value

        if value == '':
            return None

        try:
            pth, pk = value.split("|", 2)

            pth_parts = pth.split(".")
            module_path, model = pth_parts[:-1], pth_parts[-1]
            module = ".".join(module_path)
            try:
                klass = getattr(__import__(module, [], [], [model]), model)
            except AttributeError:
                return None
            try:
                return klass.objects.get(pk=pk)
            except klass.DoesNotExist:
                return None
        except ImportError:
            raise ValidationError('Could not import the model: %s' % value)