import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker

engine = sa.create_engine(
    r"mssql+pyodbc://@localhost:49242/myDb?driver=ODBC+Driver+17+for+SQL+Server"
)

with engine.connect() as conn:
    for tb_name in ["tbl_a", "tbl_c", "tbl_b"]:
        conn.execute(sa.text(f"DROP TABLE IF EXISTS [{tb_name}]"))

Base = declarative_base()


class A(Base):
    __tablename__ = "tbl_a"
    id = sa.Column(sa.Integer(), primary_key=True)
    b_id = sa.Column(sa.Integer(), sa.ForeignKey("tbl_b.id"))
    c_id = sa.Column(sa.Integer(), sa.ForeignKey("tbl_c.id"))
    related_b = relationship("B", back_populates="related_as")
    related_c = relationship("C", back_populates="related_as")


class B(Base):
    __tablename__ = "tbl_b"
    id = sa.Column(sa.Integer(), primary_key=True)
    related_cs = relationship("C", back_populates="related_b")
    related_as = relationship("A", back_populates="related_b")

class C(Base):
    __tablename__ = "tbl_c"
    id = sa.Column(sa.Integer(), primary_key=True)
    b_id = sa.Column(sa.Integer(), sa.ForeignKey("tbl_b.id"))
    related_b = relationship("B", back_populates="related_cs")
    related_as = relationship("A", back_populates="related_c")


Base.metadata.create_all(engine)

Session = sessionmaker(bind=engine)
session = Session()

b11 = B(id=11)
c111 = C(id=111)
c111.related_b = b11

a1 = A(id=1)
b22 = B(id=22)
a1.related_c = c111
a1.related_b = b22

session.add_all([b11, a1])
session.commit()

# verify by reading tables
with engine.connect() as conn:
    print(conn.execute(sa.text("SELECT * FROM tbl_b")).fetchall())
    # [(11,), (22,)]
    print(conn.execute(sa.text("SELECT * FROM tbl_c")).fetchall())
    # [(111, 11)]
    print(conn.execute(sa.text("SELECT * FROM tbl_a")).fetchall())
    # [(1, 22, 111)]