code: def get_relationships(schema, model_field=False):
      
      relationships = [key for (key, value) in schema._declared_fields.items() if isinstance(value, Relationship)]
  
      if model_field is True:
          relationships = [get_model_field(schema, key) for key in relationships]
  
      return relationships
docs: Return relationship fields of a schema
  
      :param Schema schema: a marshmallow schema
      :param list: list of relationship fields of a schema