On Apr 20, 2012, at 8:45 AM, Eric Lemoine wrote:
> On Mon, Apr 16, 2012 at 10:49 PM, Eric Lemoine
> <eric.lemo...@camptocamp.com> wrote:
>> Hi
>>
>> I'd like to use an associationproxy for a simple many-to-one relationship:
>>
>> class Child(Base):
>> __tablename__ = 'child'
>> id = Column(Integer, primary_key=True)
>> name = Column(Unicode)
>>
>> class Parent(Base):
>> __tablename__ = 'parent'
>> id = Column(Integer, primary_key=True)
>> child_id = Column(Integer, ForeignKey('child.id')
>> child_ = relationship(Child)
>> child = association_proxy('child_', 'name', creator=)
>>
>> Now 'child' is a read-only, dictionary-like table. I never want to insert
>> new rows in this table.
>>
>> So I actually pass the following "creator" to the association_proxy
>> constructor:
>>
>> def creator(name):
>> return Session.query(Child).filter_by(name=name).first()
>>
>> That does the job for "create". But I cannot find a solution for "update".
>> On update I'd like to replace the current Child object in the Parent object
>> by a new Child object read from the 'child' table.
>>
>> Using a specific setter (in a getset_factory) does not work for me, as the
>> setter receives the Child object (and the value), not the Parent object –
>> I'd need a ref to the Parent object to be able to change its Child object in
>> child_.
>>
>> Maybe I'm doing it all wrong and using an associationproxy is not the way to
>> go for that case.
the associationproxy is good for collections but in this case I'm not sure what
you're getting versus a regular @property or @hybrid_property.
I will note that I think I do a use case a little bit similar to this, where
Parent.children is a dictionary keyed on name. But instead of sticking
Session.query() into creator, I use a recipe similar to the unique object
recipe: http://www.sqlalchemy.org/trac/wiki/UsageRecipes/UniqueObject .
If the associationproxy is doing it's job, modifying "Parent.child", as a
string, should be updating Parent._child.name with the new value. If you
wanted to replace that with "just swap this other Child in", you could do that
in a before_flush() event, not unlike the insert versions recipe:
http://www.sqlalchemy.org/trac/wiki/UsageRecipes/VersionedRows
but just sticking with @property or hybrid might be more straightforward here,
if it works.
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