- changed status to resolved
simplify using backrefs on self-referring mappers
this script is a regular parent/child tree node test. but we want "father" and "childs" to have a backref relationship. self-referring mappers, when you create the "many-to-one" relationship of the parent of a node, needs an explicit "foreignkey" parameter to give it a clue which relational direction we are dealing with. but when you define the "many-to-one" side via a backref, that relational foreignkey doesnt get sent.
from sqlalchemy import *
import sqlalchemy.attributes as attributes
engine = create_engine('sqlite://', echo=True)
class Tree(object):
def __init__(self, name='', father=None):
self.name = name
self.father = father
def __str__(self):
return '<TreeNode: %s>' % self.name
def __repr__(self):
return self.__str__()
table = Table('tree', engine,
Column('id', Integer, primary_key=True),
Column('name', String(64), nullable=False),
Column('father_id', Integer, ForeignKey('tree.id'), nullable=True),)
assign_mapper(Tree, table,
properties={
# this one works, the 'childs' backref comes out ok
'father':relation(Tree, foreignkey=table.c.id,primaryjoin=table.c.father_id==table.c.id, backref='childs')},
# this one doesnt, the "father" backref does not figure out that its "uselist=False"
# 'childs':relation(Tree, foreignkey=table.c.father_id, primaryjoin=table.c.father_id==table.c.id, backref='father')},
)
table.create()
root = Tree('root')
child1 = Tree('child1', root)
child2 = Tree('child2', root)
child3 = Tree('child3', child1)
objectstore.commit()
print root.childs
print child1.childs
print child2.childs
print child2.father
print child3.father
users should have a clear path to creating mappers like this. you can, instead of using 'backref="foo"', set up the backreference relationship explicitly by defining both the one-to-many and the many-to-one relationships explicitly, and then adding "attributeext=attributes.GenericBackrefExtension(<backrefname>)". so theres ways to make backrefernces more methodically. but we should make those ways simpler, more obvious, and better documented.
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committed in changeset:1107 a new backref() function that takes the keyname as well as **kwargs to use in the new backref relation. this can be used in place of "backref='name'", i.e.