marshmallow - include_fk fail if foreign_key is not int - serialization

Question

On serializing a related database entity with sql-alchemy and marshmallow I encountered following issue:
On dumping this schema the debugger raises a ValueError with the message
self._serialize(d, many=False)
value = field_obj.serialize(attr_name, obj, accessor=self.get_attribute)
return self._serialize(value, attr, obj, **kwargs)
ret = self._format_num(value) # type: _T
return self.num_type(value)
ValueError: invalid literal for int() with base 10: 'sub'
It seems the library tries to cast the the key into an Integer. For readability reasons the key is a String in this case, so the cast obviously fails.
Ist there a flag to avoid casting the foreign_key?
Here the models for reference:
Parent Class
class Operation(db.Model):
__tablename__ = "operation"
key = db.Column(db.String(64), primary_key=True)
label = db.Column(db.String(128), nullable=False)
rules = db.relationship('app.models.rule.Rule', backref="operation")
Child Class
class Rule(db.Model):
__tablename__ = 'rule'
id = db.Column(db.Integer, primary_key=True, autoincrement="auto")
operation_key = db.Column(db.Integer, ForeignKey('operation.key'))
class RuleSchema(ma.SQLAlchemyAutoSchema):
class Meta:
model = Rule
include_fk = True

Answer 1

Found a workaround. Adding a nested field with an only condition for the schema. Api. The serialised json nests the foreign key into a object. 'operation': {'key': 'sub'}
class RuleSchema(ma.SQLAlchemyAutoSchema):
class Meta:
model = Rule
# include_fk = True
operation = ma.Nested(OperationSchema, only=('key',))