sqlalchemy postgresql json data

model.py

from sqlalchemy import Column, String, ForeignKey
from sqlalchemy.orm import relationship

from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class City(Base):
    __tablename__ = "citys"
    city = Column(String(20), unique=True)
    city_id = Column(String(6), primary_key=True)
    father_id = Column(String(6))
    areas = relationship('Area', cascade='delete', backref="city")


class Area(Base):
    __tablename__ = "areas"
    area = Column(String(50), unique=True)
    area_id = Column(String(6), primary_key=True)
    father_id = Column(ForeignKey('citys.city_id'))

sql_data.sql

COPY citys (city, city_id, father_id) FROM stdin;
郑州市	410100	410000
开封市	410200	410000
\.
COPY areas (area, area_id, father_id) FROM stdin;
郑州市	410101	410100
二七区	410102	410100
中原区	410103	410100
惠济区	410104	410100
管城区	410105	410100
金水区	410106	410100
上街区	410107	410100
郑东新区	410108	410100
航空港区	410109	410100
综合保税区	410110	410100
高新技术开发区	410111	410100
国家经济技术开发区	410112	410100
荥阳市	410113	410100
登封市	410114	410100
新密市	410115	410100
新郑市	410116	410100
中牟县	410117	410100
开封市	410201	410200
鼓楼区	410202	410200
祥符区	410203	410200
龙亭区	410204	410200
金明区	410205	410200
禹王台区	410206	410200
开封新区	410207	410200
顺河回族区	410208	410200
杞县	410209	410200
通许县	410210	410200
尉氏县	410211	410200
\.

sqlalchemy_pg_json.py

from sqlalchemy.dialects import postgresql
from sqlalchemy.sql import column
from sqlalchemy.sql.functions import func 
from sqlalchemy import create_engine
from models import City, Area


session = create_engine("")
from sqlalchemy import select
# using text method 
stmt = select([City.city, Area.area, City.city_id, Area.area_id]).where(City.city_id == Area.father_id).alias('t')
t = select([text("json_agg(json_build_object('city_id', city_id, 'area_id', area_id)), city")]).select_from(stmt).group_by(stmt.c.city)
r = session.execute(t).scalar()
print(r)

from sqlalchemy.sql.functions import GenericFunction
class json_agg(GenericFunction):
    type = postgresql.JSON

class json_object_agg(GenericFunction):
    type = postgresql.JSON

class json_build_object(GenericFunction):
    type = postgresql.JSON

t = select([City.city, City.city_id]).alias("ttt")
t = select([func.row_to_json(column('ttt'))]).select_from(t)
print(t)
# using GenericFunction
t = select([func.json_agg(func.json_build_object(stmt.c.area, stmt.c.area_id)), stmt.c.city]).select_from(stmt).group_by(stmt.c.city)
print(t)