Using attribute with RelatedList doesn't work

[AbdealiLoKo]

When I am using a RelatedList with an attribute= loading the relationship seems to cause an error about unknown attributes.

For example, if the Models are:

class School(Base):
    __tablename__ = "school"
    id = sa.Column("school_id", sa.Integer, primary_key=True)
    name = sa.Column(sa.String(255), nullable=False)

class Teacher(Base):
    __tablename__ = "teacher"
    id = sa.Column(sa.Integer, primary_key=True)

    current_school_id = sa.Column(
        sa.Integer, sa.ForeignKey(School.id), nullable=True
    )
    current_school = relationship(School, backref=backref("teachers"))

My schema is defined as:

class SchoolSchema(SQLAlchemySchema):
    class Meta:
        model = models.School
        sqla_session = session
        load_instance = True

    all_teachers = RelatedList(Related(), attribute="teachers")

Now when I try to use is:

schema = SchoolSchema()
out_school = schema.load({"all_teachers": [1]})

I get the error:

AttributeError: type object 'School' has no attribute 'all_teachers'

[AbdealiLoKo]

After creating the PR - I saw that there is already a test test_model_schema_with_attribute

marshmallow-sqlalchemy/tests/test_model_schema.py

Lines 393 to 404 in 7cdb69f

	def test_model_schema_with_attribute(
	self, models, schemas, school, student, session
	):
	class StudentSchema(Schema):
	id = fields.Int()
	class SchoolSchema(ModelSchema):
	class Meta:
	model = models.School
	sqla_session = session
	students = RelatedList(Related(attribute="students"), attribute="students")

where it shows that I would need to do:

all_teachers = RelatedList(Related(attribute="teachers"), attribute="teachers")

But I'm not sure if that is intuitive, because the attribute should be for the main field in the schema ?
Ideally the RelatedList should pass that onto the inner child during automatically ?

I see that this isn't a problem when using field_for() or auto_field() because it adds the same set of kwargs to both the inner field and RelatedField:

marshmallow-sqlalchemy/src/marshmallow_sqlalchemy/convert.py

Lines 141 to 149 in 7cdb69f

	field_kwargs.update(kwargs)
	ret = field_class(**field_kwargs)
	if (
	hasattr(prop, "direction")
	and self.DIRECTION_MAPPING[prop.direction.name]
	and prop.uselist is True
	):
	ret = RelatedList(ret, **kwargs)
	return ret

[AbdealiLoKo]

Seems like I have another issue with RelatedList and field_for() with RelatedList.
If I add a custom validator to a field like:

field_for(School, 'teachers', validate=lambda x: len(set(x)) == len(x))

It throws an error that Teacher is not itearable. Becuase this validator is also being applied to both: RelatedList and Related.

@sloria - Wanted to check if it makes sense and whether a fix to decouple the kwargs for RelatedList and Related would be accepted ?
I can attempt to create a PR

[sloria]

@AbdealiJK Yeah, that does look like a a bug--validate should apply to RelatedList and not Related. A PR would be great!