Thank you for responding, that's extremely helpful. I'm still failing to
query against it though. How would I implement a query for the
get_children() method in the example above? That is, for any given
instance of FlowBlueprint how do I get all objects with a relation to it?
On Monday, October 28, 2013 6:56:40 AM UTC-5, Daniel Roseman wrote:
>
> 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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