I ask this question here because, in my searches, this error has been generally related to queries rather than ForeignKey assignment.
The error I am getting occurs in a method of a model. Here is the code:
class Deal(models.Model): ...model_fields... def _update_existing_deal(self, deal_dict): #deal made from deal_dict here, psuedo code below deal = Deal(deal_dict) HistoricalDeal().create_historical_deal(deal) self.price = deal_dict.get('price', self.price) if self.comment != deal_dict.get['comment']: self.comment = deal_dict.get('comment', '') self.set_unit_price() logger.debug( 'Existing deal, (pk: %d), updated.', self.pk ) class HistoricalDeal(models.Model): deal = models.ForeignKey(Deal) created_at = models.DateTimeField(auto_now_add=True) price = models.DecimalField(max_digits=8, decimal_places=2, blank=True, default=0) unit_price = models.DecimalField(decimal_places=2, max_digits=6, null=True, blank=True) def create_historical_deal(self, deal): self.deal = deal self.price = deal.price self.unit_price = deal.unit_price self.save() logger.debug( 'HistoricalDeal created for Deal with pk: %d.', deal.pk ) def __str__(self): return ', '.join([self.deal.name, self.created_at.date()]) The thing is, the Deal I am passing to HistoricalDeal.create_historical_deal is legit. Here's a picture of the debugger in PyCharm. 
For search engines, the message there is:
Unable to get repr for <class 'deals.models.HistoricalDeal'> Any ideas?
Edit: Full code for Deal below:
class Deal(models.Model): LUMBER = 'lumber' WOODBLANK = 'woodblank' DOWEL = 'dowel' VENEER = 'veneer' PLYWOOD = 'plywood' TYPE_CHOICES = ( (LUMBER, 'Lumber'), (WOODBLANK, 'Wood Blank'), (DOWEL, 'Dowel'), (VENEER, 'Veneer'), (PLYWOOD, 'Plywood'), ) # define the correct method and unit for each material type # mainly used in `get_unit_price` MATERIAL_MAPPING = { LUMBER: { 'method': lambda self: float(self.price) / (float(self.board_feet) or 1), 'unit': 'BF', }, WOODBLANK: { 'method': lambda self: self.price, 'unit': 'Purchase', }, DOWEL: { 'method': lambda self: float(self.price) / (float(self.linear_feet) or 1), 'unit': 'LF', }, VENEER: { 'method': lambda self: float(self.price) / (float(self.square_feet) or 1), 'unit': 'SF', }, PLYWOOD: { 'method': lambda self: float(self.price) / (float(self.square_feet) or 1), 'unit': 'SF', } } name = models.CharField(max_length=200) slug = models.SlugField(max_length=100) url = models.CharField(max_length=200, blank=True) vendor = models.ForeignKey('Vendor') category = models.ForeignKey('Category') active = models.BooleanField(default=True) created_at = models.DateTimeField(auto_now_add=True) updated_at = models.DateTimeField(auto_now=True) price = models.DecimalField(max_digits=8, decimal_places=2, blank=True, default=0) comment = models.TextField(blank=True) img = models.ImageField(blank=True) unique_lot = models.IntegerField(default=None, blank=True, null=True) material_type = models.CharField(max_length=9, choices=TYPE_CHOICES) # attributes specific to material types board_feet = models.DecimalField(decimal_places=2, max_digits=6, null=True, blank=True) thickness = models.CharField(max_length=15, null=True, blank=True) length = models.CharField(max_length=15, null=True, blank=True) width = models.CharField(max_length=15, null=True, blank=True) diameter = models.CharField(max_length=15, null=True, blank=True) linear_feet = models.DecimalField(decimal_places=2, max_digits=6, null=True, blank=True) square_feet = models.DecimalField(decimal_places=2, max_digits=6, null=True, blank=True) adhesive_backing = models.NullBooleanField(default=False, null=True, blank=True) image = models.ForeignKey('Image', null=True, blank=True) unit_price = models.DecimalField(decimal_places=2, max_digits=6, null=True, blank=True) def set_unit_price(self): method = self.MATERIAL_MAPPING[self.material_type]['method'] self.unit_price = method(self) self.save() @property def get_unit_price(self): method = self.MATERIAL_MAPPING[self.material_type]['method'] unit = self.MATERIAL_MAPPING[self.material_type]['unit'] return { 'value': method(self), 'units': unit } @classmethod def _find_matching_deal(cls, deal_dict): """ Check for an existing deal that matches `deal_dict` """ # TODO: use get_or_create? match = cls.objects.filter( material_type=deal_dict.get('deal_type', None), board_feet=deal_dict.get('boardfeet', None), thickness=deal_dict.get('thickness', None), length=deal_dict.get('length', None), width=deal_dict.get('width', None), diameter=deal_dict.get('diameter', None), linear_feet=deal_dict.get('linear_feet', None), square_feet=deal_dict.get('square_feet', None), adhesive_backing=deal_dict.get('adhesive_backing', None), unique_lot=deal_dict.get('unique_lot', None), category=deal_dict['category'], url=deal_dict['url'] ) if not match: return None # Because of the unique constraint, there should only be one match assert len(match) == 1 return match[0] @staticmethod def _guess_category(name, url): """ Find the category that best matches the deal name/url """ name = name.lower() url = url.lower() # create a string of unique possible name variants search_string = '|'.join({ name, name.replace(' ', ''), name.replace('_', ' '), name.replace('-', ' '), name.replace('_', ''), name.replace('-', ''), url.replace(' ', ''), url.replace('-', ' '), url.replace('_', ' '), url.replace('-', ''), url.replace('_', ''), }) # TODO: cache categories, don't query each time all_categories = Category.objects.all() # get a list of categories that might match matching_categories = [ category for category in all_categories if category.name.lower() in search_string ] logger.debug('Found these category matches for %s: %s', name, matching_categories) if len(matching_categories) == 0: matching_categories = [ category for category in all_categories if category.name.replace(' ', '').lower() in search_string ] if len(matching_categories) == 0: # add it to the Misc category return Category.objects.get_or_create( name="Miscellaneous", defaults={'slug': 'misc'} )[0] # return the first match return matching_categories[0] @staticmethod def _get_vendor(vendor_name): return Vendor.objects.get_or_create( name=vendor_name, defaults={'shipping': False} )[0] @staticmethod def _capitalize_name(name): return name.replace('-', ' ').replace('_', ' ').title() def _update_existing_deal(self, deal_dict): self.price = deal_dict.get('price', self.price) if self.comment != deal_dict.get['comment']: self.comment = deal_dict.get('comment', '') self.set_unit_price() logger.debug( 'Existing deal, (pk: %d), updated.', self.pk ) @classmethod def save_from_dict(cls, deal_dict): logger.debug('saving deal from dict: %s', deal_dict) deal_dict['category'] = cls._guess_category(deal_dict['name'], deal_dict['url']) deal_dict['name'] = cls._capitalize_name(deal_dict['name']) existing_deal = cls._find_matching_deal(deal_dict) if not existing_deal: logger.debug('This is a new deal, saving it') current_deal = cls.objects.create( name=deal_dict.get('name'), slug=deal_dict.get('slug', ''), url=deal_dict.get('url'), image=Image.from_url(deal_dict.get('image_url', None)), price=deal_dict.get('price'), comment=''.join(deal_dict.get('comment', [])), material_type=deal_dict.get('deal_type', None), board_feet=deal_dict.get('boardfeet', None), thickness=deal_dict.get('thickness', None), length=deal_dict.get('length', None), width=deal_dict.get('width', None), diameter=deal_dict.get('diameter', None), linear_feet=deal_dict.get('linear_feet', None), square_feet=deal_dict.get('square_feet', None), adhesive_backing=deal_dict.get('adhesive_backing', None), unique_lot=deal_dict.get('unique_lot', None), category=deal_dict['category'], vendor=cls._get_vendor(deal_dict['vendor_name']), ) current_deal.set_unit_price() else: logger.debug( 'Existing deal, updating it (pk: %d)', existing_deal.pk ) HistoricalDeal().create_historical_deal(existing_deal) existing_deal._update_existing_deal(deal_dict) def __str__(self): return '<Deal: %d, %s>' % (self.pk, self.name) class Meta(object): unique_together = (( 'material_type', 'board_feet', 'thickness', 'length', 'width', 'diameter', 'linear_feet', 'square_feet', 'adhesive_backing', 'unique_lot', 'category', 'url' ),)