from django.forms.models import model_to_dict
import django.db.models.fields.related


class DiffableModel(object):
    """
    Make it possible to diff a model.
    """

    def __init__(self, *args, **kwargs):
        super(DiffableModel, self).__init__(*args, **kwargs)
        self.__initial = self._dict

    @property
    def diff(self):
        manytomanyfieldnames = [f.name for f in self._meta.many_to_many]
        d1 = self.__initial
        d2 = self._dict
        diffs = dict([(k, (v, d2[k])) for k, v in d1.items() if v != d2[k]])
        # Foreign key lookups
        for k, v in diffs.items():
            if type(self._meta.get_field(k)) is django.db.models.fields.related.ForeignKey:
                # If it's a foreign key, look up the name again on ourselves.
                # Since we only care about the *new* value, it's easy enough.
                diffs[k] = (v[0], getattr(self, k))
        # Many to many lookups
        if hasattr(self, 'map_manytomany_for_diff'):
            for k, v in diffs.items():
                if k in manytomanyfieldnames and k in self.map_manytomany_for_diff:
                    # Try to show the display name instead here
                    newvalue = getattr(self, self.map_manytomany_for_diff[k])
                    diffs[k] = (v[0], newvalue)
        return diffs

    def save(self, *args, **kwargs):
        super(DiffableModel, self).save(*args, **kwargs)
        self.__initial = self._dict

    @property
    def _dict(self):
        fields = [field.name for field in self._meta.fields]
        fields.extend([field.name for field in self._meta.many_to_many])
        return model_to_dict(self, fields=fields)