ValidationError from Composite Key in one-to-many relationship

[AndreasJJ]

Hello! I am having some problems with using a composite key in a one-to-many relationship table.

I have a model 'Character' with a field "alternative_name" which is a list of strings which require a relationship to another table 'CharacterAlternativeName'. This new table consists of the id of the 'Character' and the "alternative_name", both of these make up a composite key for the new table.

class CharacterAlternativeName(db.Model):
  __tablename__ = "character_alternative_name"
  character_id = sa.Column(sa.Integer, sa.ForeignKey("character.id"), primary_key=True)
  alternative_name = sa.Column(sa.String, primary_key=True)

  def __repr__(self):
      return "<CharacterAlternativeName(alternative_name={self.alternative_name!r})>".format(self=self)

class Character(db.Model):
  __tablename__ = "character"
  id = sa.Column(sa.Integer, primary_key=True)
  name = sa.Column(sa.String)
  original_name = sa.Column(sa.String)
  alternative_name = relationship("CharacterAlternativeName")
  
  def __repr__(self):
      return "<Character(name={self.name!r})>".format(self=self)

class CharacterSchema(SQLAlchemySchema):
    class Meta:
        model = Character
        include_relationships = True
        load_instance = True

    id = auto_field()
    name = auto_field()
    original_name = auto_field()
    alternative_name = auto_field()

Loading in a object such as

{
    "name": "Emma",
    "original_name": "Emily",
    "alternative_name": ["Em", "E"]
}

does however not work and gives the following validation error marshmallow.exceptions.ValidationError: {0: {'alternative_names': {0: ["Could not deserialize related value 'Em'; expected a dictionary with keys ['character_id', 'alternative_name']"]}}}.