Take self-referential many-to-many for a spin
http://www.sqlalchemy.org/docs/orm/relationships.html#self-referential-many-to-many-relationship
SRMM vs SR1M
~~~~~~~~~~~~~
Self referential one-to-many example ... familiar tree structure
* unix "Directory" has single parent "Directory", and multiple child "Directory"
Self referential many-to-many example for "People"
* some have friends (symmetrical relationship)
* some have idols (asymmetrical relationship)
* others have stalkers (asymmetrical relationship)
subsets and supersets with all manner of possible containments
.. code-block: ipython
n [2]: from NonDbi import Node
In [3]: n = Node("color")
In [5]: n.
n.__class__ n.__format__ n.__mapper__ n.__reduce_ex__ n.__str__ n.__weakref__ n.id n.right_nodes
n.__delattr__ n.__getattribute__ n.__module__ n.__repr__ n.__subclasshook__ n._decl_class_registry n.label
n.__dict__ n.__hash__ n.__new__ n.__setattr__ n.__table__ n._sa_class_manager n.left_nodes
n.__doc__ n.__init__ n.__reduce__ n.__sizeof__ n.__tablename__ n._sa_instance_state n.metadata
In [5]: n.left_nodes
n.left_nodes
In [5]: n.left_nodes
Out[5]: []
In [6]: n.left_nodes.append(Node("red"))
In [7]: n.left_nodes.append(Node("green"))
In [8]: n.left_nodes.append(Node("blue"))
In [9]: n.left_nodes
Out[9]: [Node('red'), Node('green'), Node('blue')]
In [10]: session
Out[10]: <sqlalchemy.orm.session.Session object at 0x94cdb6c>
In [11]: session.add(n)
In [12]: session.commit()
Note that the colors are not to session individually, they come
via their association::
mysql> select * from node_to_node ;
+--------------+---------------+
| left_node_id | right_node_id |
+--------------+---------------+
| 3 | 2 |
| 4 | 2 |
| 5 | 2 |
+--------------+---------------+
3 rows in set (0.00 sec)
mysql> select * from node ;
+----+-------+
| id | label |
+----+-------+
| 1 | root |
| 2 | color |
| 3 | red |
| 4 | green |
| 5 | blue |
+----+-------+
5 rows in set (0.00 sec)
Compare with Django recursive M2M to self
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* https://docs.djangoproject.com/en/dev/ref/models/fields/#django.db.models.ManyToManyField.symmetrical
Django symetrical m2m to self::
class Person(models.Model):
friends = models.ManyToManyField("self")
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.
Presumably that means can do:
class Person(models.Model):
friends = models.ManyToManyField("self")
idols = models.ManyToManyField("self", symetrical=False )
