Sqlmodel orm object no attribute xxx error for a relationship attribute ONLY IN AWS LAMBDA FUNCTION

Question

The following is my definition of a sqlmodel orm class and its related orm classes:

class CoursePostgresORM(CourseWithMetadataBase, table=True):
    __tablename__ = '...'
    __table_args__ = {'schema': '...', }
    id_: str = Field(alias='id', sa_column=Column('id', primary_key=True))
    ....

    skills: List['CourseSkillLink'] = Relationship(back_populates='course')
    regions: List['CourseRegionLinkPostgresORM'] = Relationship(back_populates='course')

    class Config:
        allow_population_by_field_name = True
    def to_elasticsearch_doc(self):
        skills = [skill for skill in self.skills]
        doc = {
            ...
        }
        return doc

class CourseSkillLink(SQLModel, table=True):
    __tablename__ = '...'
    __table_args__ = {'schema': '...', }
    id_: int = Field(alias='id', sa_column=Column('id', primary_key=True))
    course_id: str = Field(foreign_key='course_catalog.courses.id')
    skill_id: int = Field(foreign_key='public.skills.id')
    created_at: datetime
    updated_at: datetime

    course: 'CoursePostgresORM' = Relationship(back_populates='skills')
    skill: 'SkillPostgresOrm' = Relationship(back_populates='courses')

class SkillPostgresOrm(SQLModel, table=True):
    __tablename__ = 'skills'
    __table_args__ = {'schema': 'public'}
    id_: int = Field(alias='id', sa_column=Column('id', primary_key=True))
    skill_level: int
    parent_id: Optional[int]
    deactivated_at: Optional[datetime]
    created_at: Optional[datetime]
    updated_at: Optional[datetime]
    last_updated_by: Optional[str]
    name: str
    is_selected_skill: bool

    courses: List['CourseSkillLink'] = Relationship(back_populates='skill')

When run the following code on my local, everything went fine.

with Session(POSTGRES) as session:
    res = session.exec(course_stm)

However, I was thrown with the following error while running this in one of the aws lambda functions.

  "errorMessage": "'CoursePostgresORM' object has no attribute 'skills'",
  "errorType": "AttributeError",
  "stackTrace": [
    "  File \"/var/task/index_to_es.py\", line 21, in lambda_handler\n    courses = fetch_courses_from_sql()\n",
    "  File \"/var/task/index_to_es.py\", line 38, in fetch_courses_from_sql\n    es_docs = [row.to_elasticsearch_doc() for row in res.all()]\n",
    "  File \"/var/task/index_to_es.py\", line 38, in <listcomp>\n    es_docs = [row.to_elasticsearch_doc() for row in res.all()]\n",
    "  File \"/opt/python/appliedx_data_object/course.py\", line 196, in to_elasticsearch_doc\n    skills = [skill for skill in self.skills]\n"

I've spent the whole day trying to figure this out but had no luck. Any input is welcome. Thanks