On Monday, 28 October 2013 07:29:39 UTC, Joshua Orvis wrote:
> I'm using Django 1.6 RC1 and Python 3.3 with the following models:
>
> class StepBlueprint(models.Model):
> parents = models.ManyToManyField('self', blank=True, null=True,
> related_name='children')
> name = models.CharField( max_length=100 )
>
> def __str__(self):
> return self.name
>
> class FlowBlueprint(StepBlueprint):
> description = models.TextField()
>
> def get_children(self):
> print("DEBUG: processing ({0})".format(self))
> print(dir(self))
>
> # why isn't this working?? this is what related_name is supposed
> to provide
> return self.children
>
> When I call get_children() here, I get the following:
>
> DEBUG: processing (Prodigal)
> ['DoesNotExist', 'MultipleObjectsReturned', 'TYPES', '__class__',
> '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__',
> '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__',
> '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__',
> '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__',
> '__weakref__', '_base_manager', '_default_manager', '_deferred',
> '_do_insert', '_do_update', '_get_FIELD_display',
> '_get_next_or_previous_by_FIELD', '_get_next_or_previous_in_order',
> '_get_pk_val', '_get_unique_checks', '_meta', '_perform_date_checks',
> '_perform_unique_checks', '_save_parents', '_save_table', '_set_pk_val',
> '_state', 'build', 'clean', 'clean_fields', 'commandblueprint',
> 'conditional_code', 'date_error_message', 'delete', 'description',
> 'flow_set', 'flowblueprint', 'full_clean', 'get_children', 'get_command',
> 'get_type_display', 'id', 'name', 'objects', 'parents', 'pk',
> 'prepare_database_save', 'save', 'save_base', 'serializable_value',
> 'standalonetool_set', 'stepblueprint_ptr', 'stepblueprint_ptr_id', 'type',
> 'unique_error_message', 'validate_unique']
> Traceback (most recent call last):
> [ call stack ... ]
> return self.children
> AttributeError: 'FlowBlueprint' object has no attribute 'children'
>
> What am I doing wrong here? From within the FlowBlueprint object I want
> to be able to get the backwards relation of all those objects pointing to
> it, and it seems like related_name is supposed to provide that. Its value
> doesn't show up in the dir() output.
>
> Any help would be greatly appreciated.
>
This has nothing to do with inheritance - you would get the same result if
you called `self.children` from StepBlueprint.
See the documentation on many-to-many relationships with self:
https://docs.djangoproject.com/en/1.5/ref/models/fields/#django.db.models.ManyToManyField.symmetrical
(emphasis added):
"When Django processes this model, it identifies that it has a
ManyToManyField on itself, and as a result, it **doesn’t add a person_set
attribute to the Person class**. Instead, the ManyToManyField is assumed to
be symmetrical – that is, if I am your friend, then you are my friend.
If you do not want symmetry in many-to-many relationships with self, set
symmetrical to False. This will force Django to add the descriptor for the
reverse relationship, allowing ManyToManyField relationships to be
non-symmetrical."
--
DR.
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