Inconsistent behaviour of load_instance with nested schemas

[zedrdave]

Thanks to this MR, it is now possible to define load_instance at the instance level (MySchema(load_instance=False)).

However, as far as I can tell, this setting is not inherited by nested schemas, and there is no easy way to make the nested schema behave like its parent… Resulting in half-loaded schemas (top-level is a dict, with instances underneath).

Could we consider changing that, or is there an obvious workaround that I am missing?

[AbdealiLoKo]

I'm not sure what inherited by nested schemas exactly means

If I understand this right - you should just be able to do:

prop = fields.Nested(MySchema(load_instance=False))

Similar to: https://marshmallow.readthedocs.io/en/stable/nesting.html#specifying-which-fields-to-nest

[zedrdave]

@AbdealiJK you are able to define a nested schema as explicitly load_instance=False.

But I am talking about the use-case where you have a schema with a nested schema:

class Album(ma.Schema):
    name = ma.fields.String()
    artist = ma.fields.Nested(ArtistSchema)

In this case, if you use Album(load_instance=False) to get a non-loading version of the schema, the nested artist field will still be loading. If you add the load_instance=False into the schema declaration, then you lose the whole flexibility of choosing which version you want at instantiation time.

Right now, I am working around this, by manually setting nested fields:

            UpdateSchemaInst = SchemaCls(partial=True, load_instance=False)
            for key, field in UpdateSchemaInst.declared_fields.items():
                if isinstance(field, ma.fields.Nested) and not field.dump_only:
                    field.nested = field.nested(load_instance=False)

but it seems like this should be handled by the mix-in, no?

[AbdealiLoKo]

I see, got it - I recall some similar discussions regarding the partial kwarg too
See: marshmallow-code/marshmallow#438

I stopped following the thread midway - and don't know the current implementation approach it took
But just referencing it here for anyone looking to work on this

[zedrdave]

Thanks, that makes good sense: we very likely would want to support nested load_instance exactly the same way as partial…

[Aesonus]

For anybody who stumbles on this issue, I was able to work around it by using this Nested class:

from marshmallow_sqlalchemy import fields

class Nested(fields.Nested):
    """Nested field that inherits the load_instance attribute from its parent."""

    def _deserialize(self, *args, **kwargs):
        if hasattr(self.schema, "_load_instance"):
            self.schema._load_instance = self.root._load_instance
        return super()._deserialize(*args, **kwargs)

You may have to tweak it to your specific use case.