I tested on class level and the inspect function, sth like this:
inspect(FilmNode).add_property("children_lazy", relation(
Film,
remote_side=[FilmNode.id],
lazy='dynamic',
uselist=True
)
)
and
class FilmNode(Node):
children_lazy = relation(
'Film',
remote_side=[id],
lazy='dynamic',
uselist=True
)
It worked both way, I prefer the class level definition :-). By the way,
the *Film* and *FilmNode* are both sub class of Node, and FilmNode has one
--> many relationship to Film.
Thanks Michael!!
On Monday, May 11, 2015 at 11:23:17 PM UTC+8, Michael Bayer wrote:
>
>
>
> On 5/11/15 11:16 AM, Geo wrote:
>
> Ok, I got this working by adding the following code into the base class:
>
> parent_lazy = relation(
> 'Node',
> remote_side=[id],
> backref=backref(
> 'children_lazy',
> collection_class=ordering_list('position'),
> order_by=[position],
> cascade='all',
> lazy='dynamic' )
> )
>
>
> The I can use node.children_lazy to get a dynamic loader. I still have
> one question, am I able to add this dynamic loader by sub classing the
> Node? As the base class code is from the upstream, I'm wondering if I can
> have this feature without touching the base class.
>
> If the Node is mapped using the declarative system, then subclassing it is
> going to create a new mapper which links to the original using single-table
> mapper inheritance. You might be able to do that, though this would have
> the wrinkle that the existing self-referential relationships on Node would
> still return the superclass that does not have your attribute.
>
> More directly would be to add the new property to the original mapper, but
> that adds it to that mapping globally, the way to do that which is agnostic
> of whether or not declarative is used is
> "inspect(Node).add_property("children_lazy", relationship(Node, ...))".
>
>
>
>
>
>
> On Monday, May 11, 2015 at 10:54:24 PM UTC+8, Michael Bayer wrote:
>>
>>
>>
>> On 5/11/15 5:50 AM, Geo wrote:
>>
>> I have a base class node:
>>
>> class Node:
>>
>> parent = relation(
>>
>> 'Node',
>>
>> remote_side=[id],
>>
>> backref=backref(
>>
>> 'children',
>>
>> collection_class=ordering_list('position'),
>>
>> order_by=[position],
>>
>> cascade='all'
>>
>> )
>>
>> )
>>
>> by default the node.children give me a eager loading collection, and
>> what I want is to have a subclass of the Node but only change the loading
>> to lazy='dynamic' so on the sqlalchemy.orm.dynamic.AppenderQuery I can
>> control the loading on the runtime. Or If I can work on the instance level?
>> I read the documentation but I can't figure out setting the behaviour for
>> my case.
>>
>> eager or lazy loading can be controlled using query loader options,
>> however the "dynamic" loader specifically isn't compatible with loader
>> options because it changes the behavior of the attribute at the instance
>> level using an alternate descriptor. Just build a separate relationship
>> that has "dynamic" as the loader strategy and use that one when you want
>> that behavior.
>>
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