Hi Mike,
Thanks a lot for taking the time to reply…
Indeed, I came to a similar conclusion and worked out what seems like a
fairly clean way to copy relationships (assuming they do not use
secondary_join).
The fact that I cannot seem to update the mapper before the class is
created makes my code a bit more convoluted than I'd like… But this seems
to work:
def copy_relationship(source_model, relationship_name):
source_relationship = getattr(source_model.__mapper__.relationships,
relationship_name, None)
kwargs = {}
overlaps = [relationship_name]
for param_name in inspect.signature(relationship).parameters:
if value := getattr(source_relationship, param_name, None):
if param_name in ('backref', 'back_populates'):
overlaps.append(value)
elif param_name not in ('init', 'primaryjoin'):
kwargs[param_name] = value
return relationship(**kwargs, overlaps = ','.join(overlaps))
class FooViewMeta(type(db.Model)):
def __new__(mcs, name, bases, attrs):
attrs["__table__"] = (select(Foo).with_only_columns(Foo.id).subquery())
attrs["bazs"] = copy_relationship(Foo, 'bazs')
return super().__new__(mcs, name, (db.Model,), attrs)
def __init__(cls, name, bases, attrs):
super().__init__(name, bases, attrs)
for rel_name, rel_attr in cls.__mapper__.relationships.items():
expr = getattr(rel_attr, 'primaryjoin', None)
if not expr: continue
kwargs = {'operator': expr.operator}
for attr in ('left', 'right'):
side = getattr(expr, attr)
if ('__table__' in side) and (side.table == Foo.__table__) and (side.name in
cls.__table__.c):
side = foreign(getattr(cls.__table__.c, side.name))
kwargs[attr] = side
rel_attr.primary_join = BinaryExpression(**kwargs)
class FooView(metaclass=FooViewMeta):
pass
Running the code above without providing an overlaps argument to the new
relationship, results in a relationship conflict warning:
<console>:10: SAWarning: relationship 'FooView.bazs' will copy column
anon_1.id to column baz.foo_id,
which conflicts with relationship(s): 'Baz.foo' (copies foo.id to
baz.foo_id). If this is not the intention,
consider if these relationships should be linked with back_populates, or if
viewonly=True should be applied
to one or more if they are read-only. For the less common case that foreign
key constraints are partially
overlapping, the orm.foreign() annotation can be used to isolate the
columns that should be written towards.
To silence this warning, add the parameter 'overlaps="foo"' to the
'FooView.bazs' relationship.
(Background on this warning at: https://sqlalche.me/e/20/qzyx)
As you can see in the code above, adding foreign to the join expression,
does not solve the problem.
All in all, I wish there was a more straightforward way to create models
that inherit from an existing model, while loading/accessing only a subset
of columns (basically, polymorphism, without a polymorphism key)… Will
gladly take any suggestion on a different approach…
But at least this seems to work!
Thanks again,
--
Dave
On Thursday, 7 September 2023 at 13:33:05 UTC+1 Mike Bayer wrote:
>
>
> On Thu, Sep 7, 2023, at 4:39 AM, [email protected] wrote:
>
>
> *Is there a clean way to (programmatically) duplicate all relationship
> from an existing model, over to a new model (that targets the same table
> and selects a subset of columns as a subquery)?*
>
>
> relatonships are fixed to their parent class and have internal state that
> is dependent on it, so it can't be shared between two unrelated classes.
>
> you would need to introspect the attributes of each relationship that are
> important to the application, such as the target class etc., and create a
> new relationship() that has those attributes. Assuming the relationships
> dont have special join conditions, this would involve just looking for
> existing_class.__mapper__.attrs.relationship.entity, which is the mapper to
> which it refers, and placing this into a new relationship().
>
>
>
>
>
--
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http://www.sqlalchemy.org/
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